How Electronic Signatures Work

Hello, I've come across various applications where I am needed to sign electronically: I am given a tablet or mobile phone and put in my signature (or something that resembles it at least) in a specific placeholder using either a stylus or even my finger! This is common in not-so-important things like couriers but recently I've also be instructed to sign like this in banks (!)

There seems to be some secret occult on how such solutions work. I understand they get a bpm/jpg (or whatever) with my scribble; then what? Also, I am really not sure if such solutions are legally binding! What would happen if tomorrow I say to my courier that I didn't receive the package and that ain't my signature there?

Thanks for any help!