I was on Gmail with a custom domain for a good 7 years or so and in this last year I switched to iCloud mail and have been happy with it. I trust apple more than I do google when it comes to email privacy. And I think what really pulled me over to having apple host my mail is the private relay feature which is super cool.
Both. I mainly check my email on my iPhone but on my PC I have it setup in the Outlook application (the one that’s part of MS Office). Never have any issues when I’m on the PC accessing it.