Having been a trainer for almost 25 years I absorbed a lot on the job, especially observing other trainers. Watching videos of trainers on YouTube just reiterates one interesting behavior that you need to be cautious of when working with, and evaluating, your trainer.
No trainer should be “helping” you perform the exercise. A psychological technique many trainers employ is to help/guide/steer you physically through all your sets by keeping their hands on you at all times and actually driving you as you perform the exercise.
Obviously you must do the exercise yourself to get the benefit — to build up stamina, balance and strength. Someone physically manipulating you every step of the way is saying “You can’t do this without me. You need me.”
Everyone wants to stay employed. Everyone wants to keep their clients, and giving you the idea that you can’t do it without them by habitually engaging you physically creates a psychological reliance.
A trainer guiding you now and then in order to demonstrate proper form is perfectly acceptable, but performing your set along with you as a matter of course certainly is not.