but when a computer does it... can someone explain how that works?
It creates a combination, checks that it is valid, and if it is then goes through the process of turning that combination of words in to usually either a master public key which it can compare against one you have entered, or in to an address which it compare against one you have entered or the blockchain for any previous transactions.
So you would see on the screen... it typing those word combinations and pressing enter each time... then move on to the next word etc?
I mean, you could write a program to do this if you wanted, but given that it could likely check tens of thousands combinations a second, then there really is no point.
You mean out of twelve words right? So how much is limit for the twenty four word seed?
The same. If you are missing 4 words then there are 2048
4 possible combinations, regardless of how many total words there.