I need to send/take in my Mac for repairs. How do I protect my personal information?

Back up your machine with Time Machine. Create a new Administrator account on the machine that the techs can use to boot the machine and test. You will need to give the techs the password. Delete your old account.

When you get, restore your machine from backup. If they didn’t have to replace or reformat the hard drive, you might even be able to restore from a local snapshot in30 seconds, but don’t count on that.

Protect it how— you need a password to log into your Account— so it is psswd protected.

If you use File Vault, then it is File Vault protected.

Always have a known good backup, before you send it in.

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

Boot clone https://discussions.apple.com/docs/DOC-10081

How to use Time Machine to back up or restore your Mac: https://support.apple.com/en-us/HT201250

Use DiskUtility Restore feature https://support.apple.com/guide/disk-utility/restore-a-disk-dskutl14062/mac

note: >System Preferences>Security & Privacy >Privacy>Full Disk Access

unlock the padlock, press the + button and add Disk Utility