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.