Programs that automatically open on startup?

When you restart or shutdown the Mac is "Reopen windows when logging back in" checked? If so, then any apps which are open when your restart/shutdown, then that app(s) will automatically start.

Prevent apps and windows from reopening - Apple Support

Mmm…🧐

1 - Right-click the Mail icon (when open)

2 - Go to Options

3 - Deselect "Open at Login"

A different solution: Go take a look at System Prefs>Users & Groups, choose yourself and click on the "Login Items" button. You can add/subtract what boots at startup.