if you did a full format, it got overwritten by zeros
if you did a quick format, the file allocation tables, the pointers to all the file pieces, were recreated so that the file system thinks that all the space is empty. All the files are really still sitting there until new files get written there.
on an SSD you cant overwrite data. To write to a block of flashram it first has to be zero'd. this is what garbage collection and trim do. They zero out the newly 'deleted' blocks so the drives performance stays near peak.