how do sitters access your home to check on your pet for a drop in?

Great question. The answer is: That's completely up to you. Here are some options:
Give them a spare key to your home or leave them a spare key somewhere on your property that they are to put back when they leave.
Give them your garage code, if you have one, so they can enter through the garage. Or give them a spare garage door opener while they are doing drop-ins for you.
Get a lockbox and put a spare key inside, and give them the code to the lockbox so they can get the key out and put it back when they leave.
Some owners will even remote open the garage or remote unlock a door when the sitter texts them that they have arrived.
Of course you can always leave the door unlocked, but it's certainly not the recommended option.
I hope that's helpful!