LlamaMama, that question made me check it in game and now I'm more confused than before

First, I took the same Sim that in one game used harvestables, and start checking, but he kept using regular ingredients even after maxing mixology, then gardening, and then cooking. After that I sent him to moonlight and after only one night moonlightning (I stocked that bar but didn't get a chance to use ingredients) once he was home, he was using harvestables stocked in his bar at home.
After that, I start new game with same Sim and send him to moonlight as soon as he hit level 2 (it is not possible before). But he didn't start using ingredients until level 6 mixology, and after 3 nights moonlighting. He did use it in community bars, when I stock and chose manually ingredients for next drink, and he got some nice tips and comments for those drinks.
So it is not clear for me neither, but trick is to moonlight and possibly use harvestables (or bought one) while moonlighting first and then at home. You don't need to garden, cook, learn popular recipes nor to create new drinks. I would like to hear other experiences since this is still not so clear for me.