As nail technicians we need to always keep evolving. I did my foundation course in gel enhancements with CND, and we didn't learn to infill either. I did a few separate courses with various educators and brands to learn more, like reverse French, gel babyboomers, different shapes, infills and rebalance, etc, which was never part of my foundation course. It's always valuable to keep taking courses. I urge you to do what Trinity said, invest in a 1-2-1 with a good educator, learn good ways to do rebalance, including dealing with lifting, which inevitably will happen in the beginning of your career (and even after years you can still get clients with lifting).
The gist of infill: remove old gel colour, and if needed (depending on growth) thin out part of the apex area (the goal is to rebuild the apex in the correct place, since it will have grown out at this point), push cuticles, prep the natural part of the nails etc, and place your product at the grown out area to fill this part. Then check the nail from various angles to see if you need to place more product at the apex area. Finish file, and apply new colour. If the customer wants to shorten the nails at infills, then do this, but also when you do, the free edge will be thicker where you've shortened it, so you'll need to reshape.