The mx server ip will be from the email provider with thousands of whitelisted domains. You get lumped in with the rest and cosidered safe. Spam filters use Bayesian statistics, the logic is not what you expect.

No, your emails are still sent from your providers servers and are signed by Proton/Tuta/Gmail/whatever you choose to use. Your domain is the from: field but it was sent by the actual email provider with all the trust behind it.

I never ran into something requiring a Google/Apple address. Some may require a Google account, but you can create a Google account with any email address. You don't actually need to use Gmail on your Google account.

yes but doesnt that just get flagged as spam?

If you're willing to run "mail-in-a-box", with full control of the DNS for your domain name, it sets up all of the security DNS records to not get flagged as spam.

If you're unwilling or unable to let mail-in-a-box control the full DNS for your domain, mail-in-a-box still has a page that will list every DNS record that needs add to your un-self-hosted DNS. It is many many DNS records, for best spam protection.

My other concern was receiving spam.

But the default spam filter in mail-in-a-box felt immediately noticeably (but just anecdotally) better than the one built into Proton. Which surprised me. I had assumed that I would give up some spam filter quality by self-hosting.

Custom domain without self hosting still requires you to use an email provider, could even be google. The only problem is if you self host and don't rely on trusted proxies, at which point it depends on the reputation of the IP/Domain etc.