Holding Command + R to boot the Mac will boot the laptop into Recovery Mode.
To configure macOS to use another boot drive you must select another bootable drive from the Startup Disk in System Preferences, or I believe Recovery Mode has the "Startup Disk" option on one of the menus (Apple menu maybe, don't recall which menu). This method changes the default startup disk for that Mac until you clear it with a PRAM Reset or select a different default boot drive.
Or for a one time boot option, hold the Option key immediately after the startup chime. Any bootable volumes detected will be displayed as an option (best to have a bootable USB drive already connected since after a while of sitting at the boot picker menu the Mac may not update the options if a bootable drive is connected).
Change your Mac startup disk - Apple Support
Like @BDAqua mentions you really need to be booting from macOS 10.15+ (the early 2019 13" & 15" models did support Mojave, but you will need a current version of the Mojave installer since older versions of Mojave won't be able to boot even those models). You must use an OS installer that is the same version or newer than the OS which shipped with the Mac from the factory. Not all Mojave installers will work unless they have a build version equal to or higher than the build which shipped from the factory (so download the current Mojave installer if you have the earlier 2019 models since older versions of the installer won't work even if it is "Mojave").
Plus to boot from a USB drive you first need to modify the security settings from Recovery Mode by using the Startup Security Utility.
As long as you have a good Internet connection, Internet Recovery Mode is a good option to reinstall macOS. At the very least you should be able to access the online installer for the OS which originally shipped from the factory by using Command + Option + Shift + R. Or you can try booting to Recovery Mode (Command + R) to access the installer for the OS currently installed. Or if you want the most recent version of macOS compatible with your Mac, then use Command + Option + R. These latter two options do not always work as intended since sometimes they will instead just access the online installer for the OS which originally shipped with the Mac.
With a 2018+ Mac, you also have another option if you have access to another Mac running macOS 10.15+:
Revive or restore an Intel-based Mac using Apple Configurator - Apple Support
