Hi! The game was made on Game Maker Studio 2, if you have it you can just download the source files from here https://drive.google.com/file/d/15DKxJ_eOoysryDv48Pw-5OOrXmCWhCyh/view?usp=shari... you can go nuts on them for free, no problem haha
Now to find everything about the breaking/digging you only need to open the object called oGround, the code is very simple, its a block that breaks when in contact with the shovel hitbox, and at every frame it checks if there are any empty spaces around it (which hogs memory, so you might want to fix that), if there is he changes its own sprite to fit the empty space, in total there are 15 total variations for the ground/dirt block sprite, one for the top area being empty, one for the under area, one for the right, left, up & right, up & left, etc...
I think thats about it, if you need any more explanation or the code in some other form, just leave a message and I'll try to help :)