Yeah , slugs usually chew holes in things , but plant juice suckers like aphids and thrips can leave marks like that. They can also lead to yellowing . Your basil looks yellowish , though I don't know if that is just the camera / lighting . Thrips are tiny and can be very hard to spot . Shake a leaf over a piece of paper . The thrips should be viSable on the paper as teeny black threads.
In the meantime , your basil plants are too crowded. The big one can go in the pot it will stay in, one gallon size . The babies can go in their own separate cups.
Best wishes ! Keep us posted on what you find and how your basil does .