Ctrl+Alt+F# are shortcuts for switching between TTYs.
TTY stands for TeleTYpe, it’s is the standard I/O interface for all POSIX computers. In the old days, on Unix, real teletypewriters like this beauty here, the Teletype Model 33, were used as terminals to interact with the computer:
They are also called “dumb terminals” because they are not computers in themselves, just the IO interface (usually keyboard for stdin, and line printer for stdout and stderr). Since POSIX operating systems are multi-user, multiple users can use the computer at the same time, each using his or her own teletypewriter to connect to it via telephone line. Nowadays, we don’t use actual teletypewriters, and we’ve replaced line printers with monitors, and now the terminal IO is handled by software emulated TTYs, and your EndeavourOS PC has about 70 of them (they are all in the /dev directory). You can switch to some of them using the Ctrl+Alt+F# shortcut.
If you are using KDE Plasma, and SDDM as your display manager, your GUI session will typically be on TTY1, so you would use Ctrl+Alt+F1 to switch back to your default GUI session. On some other desktop environments, the GUI session can be on TTY7.