pruning happens after downloading the core or before.
You can turn on pruning whenever you want... but note that even if you enable pruning right at the beginning... you will still need to
download the entire blockchain from the beginning... so you're still going to be downloading 250+gigs worth of block data.
It's just that it isn't stored permanently on the disk...
There are also some caveats to using a pruned node... like if you ever need to reindex (corrupted blocks) or rescan (in the event of importing a private key or "watch only" address)... you will need to redownload the entire blockchain again, so the wallet can scan all the blocks looking for transactions that involve those imported key(s) and/or address(es).