Does Time Machine back up everything on my Mac or do I need My Passport for Mac to back up everything?

Welcome to Apple Support Communities.

Time Machine will backup everything, but you need an external hard drive for Time Machine to work.

For the external drive, I would not get a Western Digital My Passport. Get a high-quality, desktop-class drive, such as the LaCie 6TB d2 Professional Desktop Hard Drive - Apple Store.

Next, format the drive as APFS for the format, and GUID as the scheme: Erase and reformat a storage device in Disk Utility on Mac - Apple Support. Finally, Back up your Mac with Time Machine - Apple Support.

-Jack

Hello, Stryker1949.

Yes, Time Machine will back up everything.

Back up your Mac with Time Machine - Apple Support:

https://support.apple.com/en-us/HT201250