I no longer make that kind of money through any side project but I did during ~ a year of college. It was one of the worst projects (coding wise) I made, and I made it all through ads.

It was just an Android soundboard app for a very popular TV show in Spain. You could tap a button and it'd play some funny sentence from a character from this show. Long press, you could share that sound.

Revenue model: a little ad bar on the bottom of the screen.

It grew from 60$/month to > 1000$ in around 6 months, only through word of mouth. That lasted another 6 months or so until the show became less trendy.

Probably not very sustainable as I was piggybacking on the popularity of the show, but it taught me that making money was 100% not about writing 'the bestest codes'.