Mac storage basics · Time Machine basics
Can I use the same external drive for Time Machine and my own files?
Short answer: Yes — add a separate APFS volume for your files and let Time Machine use its own. But if the drive fails you lose both, so back up files that live only there.
Yes, by giving them separate areas: in Disk Utility add a separate APFS volume (or partition) for your files on the same drive, and let Time Machine use its own volume. APFS volumes share space, so you don't have to fix sizes in advance.
The catch: if that one drive fails, you lose both your backup and the files stored there. For files that live only on the drive, a separate backup is wise.