Why, if I move one block, all blocks around it move too?
Blocks stick together because of "glue" on their sides. It's a very useful thing, but sometimes it becomes unwanted. To unglue a custom block, select it, tap Pencil button ✏️ and then tap on a LEGO brick button. See those white dots? Just tap on them and they will disappear, making blocks around unable to stick to your custom block.