damn, today I learned that ligatures are inappropriate in some contexts!
So ligatures are like if you write two letters, in some fonts they are merged. For example in "tt", there is just one big line crossing the ts, not two small ones for each t. But there are also ligatures for ff and other letter combinations,
As this package states (https://www.ctan.org/pkg/selnolig), it is awkward to do ligatures across morpheme boundaries. They give the example of "shelfful", which is shelf + full, so it kinda makes little sense to join the two fs in a ligature.
The more you know 💫