In vertex mode, I usually shift select in the middle of an edge and the loop will become selected, but it does nothing in 2.90.1 ?
Shift is used to extend selection.
Alt Left click is default to select an edge loop.
Double left click is default if you emulate 3 Button Mouse on a laptop or use Industry Compatible keymap.
Shortcut can be different according to circumstances, user preferences.
Check in Preferences Window, shortcut used for Loop Select operator in Keymap section.
Alt left click does nothing.
It all works properly in 2.8x (for me)
Wait just a minute, OK, yes I had ‘emulate 3 button mouse’ accidentally enabled. And yeah, it’s alt for loop select. Awesome thank you.