hey @tvguy welcome to the nextcloud community
as @wwe has suggested, you’ll find information in How to install Nextcloud snap
the snap installation will not offer alternative database selection during installation process, generally you’ll have to use the Database which comes with the snap:
However, the snap is very opinionated. Don’t want to use Apache? Sorry, the snap uses it. Don’t want to use MySQL? Sorry, that’s what the snap uses. Something other than PHP whatever-we’re-using? We picked the version we feel gives the best results. In other words, it’s not very tinker-friendly. We don’t do this to be mean, we do this so that we can reliably update it without your needing to worry about it.
you could change the data directory but there are caveats, see Backup, restore & migrate and Export & import
feel free to check out our wiki Nextcloud snap wiki