Hi @jordimas,
The "recovery mode" is part of the boot loader, similar to the PC.
It can be used to repair and/or sideload the current OEM operating system.
Fastboot is more like a "bios".
Fastboot is a "command-line tool" with many more commands than just the one «flash».
But be warned, you can destroy your device, because similar to the bios of PCs, there are different fastboot properties depending on the device manufacturer.
See also here https://technastic.com/fastboot-commands-list/#Fastboot_Commands_List
and in particular here https://technastic.com/fastboot-commands-list/#Fastboot_Command_to_Unlock_the_Bootloader
and https://technastic.com/fastboot-commands-list/#Prerequisites
finaly https://technastic.com/fastboot-commands-list/#Reboot_the_Device_into_the_Recovery_Mode
👉 You should read up more before you try it out. Above all, it should always be device-specific.
Greetings Mario