This is a long post and I have said a few of these things before.
Do you have a coach who is explaining to you how he is building your loop along the way?
I learned looping in the 38mm ball era. My Chinese coach started me learning how to loop no spin ball, on the way own, standing a bit further away from the table. Using this method, he was trying to get me to feel what it is like to brush a ball properly and "lift" it over to the other side of the table with a good arc.
Once that was done, he had me loop backspin ball coming at me, also on the way down, and lift it with slow spinny loops.
The last piece of the puzzle was him having me learn how to loop backspin, no spin and topspin at the top of the bounce. That is the loop kill or loop drive I am referring to. When the final piece of the puzzle fell into place, I had already learned how to brush properly by then.
Along the way, here is also the sequence he taught me: 1) use elbow/forearm to generate spin and once I perfected that, he added 2) waist movement/hip rotation and then 3) pushing off the floor with my legs. Those all happened in the first two phases: spin the ball with lots and lots of spin while the ball is on the way down.
During the second phase (looping underspin ball on its way down), he also asked me to "explode" or "accelerate" into the ball, regardless of how much backspin is on the ball. I remember this phase of the training very well because he basically asked me to rebuild my topspin loop at that point.
The more backspin is on the ball, the faster you need to explode with your body to lift the ball properly. Whenever a player learning how to loop backspin, the most common reason for the ball to fall into the net is that there is no to little "explosion" or "acceleration." It is like, you need to whip your whole body into the shot. The more backspin is there on the ball, the more you need to "whip" your body. In this case, timing is crucial. Basically you have to time the ball correctly, relax your body completely. When the ball comes to you (because you timed it properly), you immediately explode (from a relaxed position) and whip your whole body into the ball and finish with a follow through. the follow through should end where you want to guide the ball to. In other words, the follow through should end to your left if you are aiming for the opponent's forehand. The follow through should end to the right side of your head if you are aiming for opponent's backhand, assuming the opponent is right handed.
The problem with the last phase of building my loop was that I was confused. Up to that point, I could lift any heavy backspin balls by lightly brushing the ball and I won a lot of points in practice matches with other juniors back then (it was 38mm celluloid ball after all). So, literally one day at our session, he basically told me to stop doing that! Instead, in all of our subsequent multiball sessions, he gave me heavier and heavier backspin balls and asked me to loop or whip through ball. I was confused because wouldn't that mean the ball would hit the net and bounce back to my end of the table? He simply said, trust me and just do it! Don't ask too many questions!
Lots and lots of multi-ball sessions later, I finally got what he was referring to. It does not matter how much backspin is on the ball, just as long as you loop drive at the top of the bounce with even faster acceleration, you can loop drive/loop kill any ball (mind you, up to that point, I was taught to loop the ball on the way down. now he wanted me to loop the ball on the top of the bounce? That in itself took some time for me to adjust because timing was different and the height of the ball was different).
In the final phase (looping underspin, topspin or no spin at the top of the bounce), he also asked me to add shoulder movement into my loop and push off the ground even harder. Pushing off the ground was easy (he already taught me that in phase 1 and 2 of the training). But adding shoulder movement? I found that to be hard. Up to that point, it was already difficult enough to time the ball correctly, move my feet to where the ball should be and engage my elbow/forearm, waist/hip and my legs all the same time, while trying to be relaxed and then explode through the ball. To add shoulder movement really set me back for a few months until it became natural.
Wrist movement. He did not teach me any of that because when you are building your forehand loop, if you move your wrist by the slightest angle, your ball will fly all over the place. So he asked to lock my wrist and move my elbow, shoulder, hip and legs in one synchronous motion. I added wrist later on on my own after I got comfortable with looping.
Fang Bao has very very good YouTube videos. However, I don't think they are useful for beginners and intermediate players. The videos I have watched, it is just a bunch of experts who started playing when they were 4 or 5 year old displaying what a perfect loop looks like. It is totally useless for adult beginners. You are not going to get to a perfect loop from the get-go. You have to build the loop step by step.
Of note, every coach has a different idea on how to build a loop for your OVER TIME. It is not going to happen in 1 day or 2 weeks or even 2 months. To perfect loops, it takes years!
As for slow spinny loop, that's where I use the tacky top sheet to lift the ball. As for fast, spinny loop at the top of the bounce, I do the full motion and engage both the top sheet and the sponge.
Going back to the topic of looping a heavy backspin ball into the net. The number 1 reason again is, not enough acceleration or explosion. But in order to be able to accelerate and explode properly you need to 1) move your feet into the right place at the table, 2) account for the height of the ball (every ball coming at you will vary in heights) and 3) be TOTALLY relaxed. If you are not relaxed completely, you cannot accelerate or explode because you are already tight. Not being relaxed totally prior to initiating the looping motion seems to be the biggest problem for adult beginners because they are "tight" and they are "scared" and they are "nervous." You have to get all those fears out of your head if you want to loop with quality. You need to be completely relaxed (head included; I am semi joking or semi serious. If your head/brain is not relaxed, then your whole body is not relaxed and you are going to generate really bad loops with low quality).
I started playing ping pong seriously with a coach when I was 13. So I could see things from both a 5 or 6 year old perspective v.s. an adult beginner perspective. But that's why Fang Bo or ZJK is probably terrible at teaching adult beginners because to them, once they picked up the paddle at the age of 5 or 6, all the above were natural and instinctive for them at that age. To teach adult beginners, you need a whole set of different strategy/lesson plans. You need to build your loop over time and have the patience to get to the end.
Of note, maybe the way my coach taught me is out of date in the 40mm plastic ball era. The coaches at the club I am playing at emphasize looping on the top of the bounce or slightly below the top of the bounce. I personally don't get that. Maybe they are trying to give adult beginners confidence? I still believe that loop should be taught in the beginning with the ball on the way down because you need to learn to brush the ball and "lift" the ball over to the other side of the table. If you teach adult beginners to loop at the top of the bounce, you are basically teaching them to smash and hit the ball because that feels natural to them (hey, the ball is high, then just smack the heck out of the ball).
In other words, the answer to your question is that, your loop has not been built yet and you are already asking too many questions. When you build your loop over time, you have to trust your coach and trust the process. If you want to win at the table today with your newly developed loops, then you have lost already because you are tight, standing there. How can you loop properly when you are already tight?
