It's using a whole bunch of Steam Deck spare parts bought from iFixit, and a few after market upgrades like hall effect sticks and an extremerate shell replacement. Buying a single trigger (just the plastic R2/L2 trigger, mind you) for $20 to fix a broken $500 Deck isn't too bad, but trying to build an entire controller from spares is really not economically sensible.

But if you did indeed have those parts already for some reason, the rest is all rather cheap, common components. Cannibalize a Deck, and the extra cost would probably be well under $50.
...plus the $500 to buy a replacement Deck, so don't actually do it.

More options. It can be quite comfortable to use split controllers wirh your arms in a relaxed position.