You generally do all 3 categories regardless of the main lift. The exercises aren't always set in stone, you can do lighter push work after benching etc, but still use all 3 categories.
And single leg/core are in same category of assistance, not separated. So say you're aiming for 50 reps total per category, you would do 50 reps of your chosen single leg OR core exercise, if you normally stick to one movement each.
In that example you could do 25 reps of a single leg movement and 25 reps core, but still treating it as one category so not 50 of each.
Same for something like curls or pressdowns, you could do 15-20 total reps for them and fill out the rest with pullups, DB presses etc. Plenty of ways to fit in a variety of exercises if you want them.