Info? Because Google Apps for your domain also known as G Suite (legacy) is the method we've used for this and is going away as a free option. The other approach is to integrate a free gmail account with a mail hosting provider. You can use mailgun or similar to do it cheap but it still means you host your domain email server somewhere.

Google domains lets me do it for free.

True, my domain is also from Google domains (it's a .dev). I didn't think this feature was exclusive to that, but maybe it is.

Sounds like you might be using the free Google email forwarding feature outlined at https://domains.google/get-started/email/ To directly receive and send email from your custom domain Google upsells you to Google Workspaces https://domains.google/learn/how-to-use-email-forwarding/

Some other domain providers do offer email forwarding so its not exactly a Google exclusive.

I'm experimenting with CloudFlare's email forwarding (currently in beta) and it works very nice. Also, it's free and doesn't require changing registrars.