Ubuntu 20.04

Long story short: on a new box mysql 8 was installed, then uninstalled and purged, then mysql 5.7 was installed with a fair bit of difficulty via alternative repositories. For reference, here is a link to the recipe I used to install 5.7: https://computingforgeeks.com/how-to-install-mysql-on-ubuntu-focal/

My question: How do I ensure that future upgrade activities will preserve mysql 5.7 and avoid mysql 8 being inadvertently upgraded on top of it?

Thank you for your time.