Linux find/search root partition ONLY?

Use the -xdev argument to find

-xdev Don’t descend directories on other filesystems.

The POSIX standard defines the -xdev ”primary“:

it shall cause find not to continue descending past directories that have a different device ID

This is implemented in GNU's find (i.e. the findutils; docs).
GNU find also allows you to use -mount as ”an alternate name for -xdev, for compatibility with some other versions of find.“

On BSD systems and macOS the option is -x instead. They (imho confusingly) call -xdev to be ”deprecated“. You can, however, use -mount instead. [manpages of FreeBSD and macOS]

other tools, just FYI

If you're using ripgrep (rg) like rg --files, you can use the --one-file-system option, which does the same like find's -xdev option.