ALSA has supported the first generation of Scarlett interfaces for some time, but support for 2nd and 3rd generation devices was only added to kernel recently
(Oct 21: Scarlett 8i6 input not showing in settings)
I suggest using this command to enable the driver for Scarlet 18i20 Gen 2
sudo tee /etc/modprobe.d/scarlett.conf <<<'options snd_usb_audio vid=0x1235 pid=0x8201 device_setup=1'
Or this for Scarlet 18i20 3rd Gen
sudo tee /etc/modprobe.d/scarlett.conf <<<'options snd_usb_audio vid=0x1235 pid=0x8215 device_setup=1'
Release Scarlett Gen 2 & 3 · geoffreybennett/scarlett-gen2 · GitHub
sound/usb/mixer_scarlett_gen2.c - Linux source code (v5.15.12) - Bootlin
EDIT: removed typo in 1st command (spurious backtick)