You used to be able to get gsuite for free with your own personal domain. A lot of people, especially on HN are probably still grandfathered in on that for the free tier. Nowadays I think you have to pay for using a custom domain.
I still use it!
I have two separate "Google apps" for two domains, and I just add new domains as aliases. It allows you to separately select DKIM keys for even aliased domains.
I don't know what's the limit for aliases, but I have at least 5 so far.
I'm grandfathered into a legacy Google Apps for Your Domain account, and it's nice but there are downsides (it's ineligible for many Google services like Play Music - oh well /s)