Redeeming a Steam key is just an extra option, and it doesn't affect your ability to download/access the game from Itch.io. You can feel free to download the android version from itch while also using the key to get the Steam version on PC. This should also work for bundle purchases.
I just don't know where the key would be. As stated I haven't really gotten a steam key via ichio before lol.
Honestly, just played the demo for this one and it does make me wanna buy that bundle. This gives me some Hunniepop vibes, and I can dig it. Nice quality overall, only improvements imo would be some animations.
The key should be redeemable on the downloads page you get once you purchase it. There are a couple of guides around the internet for how to do it, like this one:
https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase