Blocks can reside inside groups and vice versa. In fact, a group can contain multiple subgroups and blocks with subblocks. Each new use of a block simply references the copy in the library. So repetitive use of a group can vastly increase drawing size. Blocks are available only to the drawing that has a library containing them.
Symbols are available to all drawings.
All this flexibility can get confusing, so set some work standards. You create a block by dragging objects from the drawing into the block palette. Changing a block automatically changes the references to the block that the drawing already contains. Blocks can be associative doors and windows understand walls, for instance. Move them, and the break in the wall moves with them.