Different randomized username on everything I sign up for. I do it so you can't google my username on one platform and find me on another. Each account also gets its own unique email address.
I personally pay the extra $1.50 for iCloud+ (due to the extra storage) and that comes with “hide my email” - which lets you generate an alias specific to the site you’re signing up on.
Then if I get sick of the site or I feel it’s getting spammy I just delete the alias
Same domain. Every email is just the username it's associated with @ the domain (Not gmail). The passwords are different between account and email (And no two accounts anywhere share passwords).
As of right now I have 19 already-created email accounts just waiting to eventually be associated with some account I'll make for some service in the future. Any time I get low I'll make a bunch more at once. I have almost 60 accounts across the internet using this system already. It does get a bit annoying when certain sites want to email me a login code every time I log in.