MacBook Pro 2019 reset to factory: Mojave or Catalina?
See Apple's documentation on Recovery Mode on ways to reinstall macOS and/or erase the disk. A simple reinstallation should not impact installed applications or user data, but as usual making a backup before is recommended. Also, downgrading from Catalina to Mojave always require changes in the disk format/partitions so you must make a backup before.
Which version of macOS is installed during Recovery can be partially influenced by the way Recovery Mode is launched (see https://support.apple.com/kb/HT204904):

Cmd-R reinstalls the same version as currently installed
Opt-Cmd-R installs the newest version available
Shift-Opt-Cmd-R installs the version the Mac shipped with (or the oldest one still available if the shipped version isn't available any longer)

If you clean install macOS, you will &quot;wipe out&quot; any FileVault encryption so whether you have the key or not is rendered moot.  The boot firmware password, however, is not affected.