What are these transformers in my garage for and how do I disconnect them?
One of them (top picture) is your door bell, and the second one could be the garage opener, sprinkler controller, or low voltage lighting of some sort.
Typically, there is an electrical box behind it and it is 120v. You can use ether multi-meter or electrical tester like this to determent if the wires are hot or not and shut one breaker at the time. Sometime there might be a label in your service panel for those.