@AlanyG21 there's no easy answer but here's the flowchart:
1. if the game is dirt cheap, buy it on steam, because fixed paypal fees on itch take too much out of cheap games
2. if the game is not dirt cheap, buy it on itch, cause the dev gets way more. if you really wanna help, leave a nice tip cause itch lets you do that
3. if you buy the game on steam, directly for yourself, and leave a positive review, that can be more valuable than the extra revenue generated by an itch sale. so if you really want to support devs and keep your library of games mostly on steam always leave a positive review of indie games you buy there. the actual review text doesn't matter so just "good game 👍" is sufficient. but remember that reviews only count for metrics and store visibility if you buy directly on steam and its not a gift