For mice it’s tunnel handling, where you just let them walk into a tube and pick them up. You do need to scruff them to hold them for actual procedures and to examine their teeth and stuff, but it’s really stressful for them to be snatched out of their home by the tail.

For rats it’s just picking them up with your hand over their back and under the armpits and then support the bum with your other hand like you would a kitten or any other small domestic critter. Rats are generally more calm and don’t mind being picked up, mice don’t love it and will jump or run away or bite.