Most likely its a lot of wood directly on wood -
Appears to be D Log siding over osb sheathing over conventional stud framing, and most likely osb decking on joists - hopefully felt is behind the D Log siding to prevent water transfer. We like to fir out the siding for an air gap.
Osb was not as moisture resistant in 1997 as it is now. Today it outperforms plywood.
First thing I noticed is the porch attached to the unprotected osb means the osb and band behind it is getting soaked. The door is likely letting water into the subfloor at the corners too.
Secondly the porch needs to be lower- it can shed water right into the door way- even a few inches can gain you decades.
The extent of damage can be from mild water stains to soft wood to termites and all about gone- and anything in between.
The screen door and porch sheds some bulk water so it may not be extremely bad-at the same time the attached porch is transferring and trapping water - I ‘d expect the lower door jambs, door threshold and subfloor below, especially in the door corners , to be pretty bad.
Worse case - You want to have a couple of days and good weather-
Then pull both doors if it appears extensive from below, remove the porch, cut the exposed osb sheathing away and fix the band if needed and the floor decking , resheathe it , then flash the new sheathing osb with ice and water shield and roll that into the opening , and seal both lower corners with Zip Flex tape- at this point I would shield the ice and water shield with thin aluminum trim coil metal folded into the door opening and hemmed back -
Then attach the porch and use stacked washers to keep it 1/2” off the aluminum so it can breathe- it needs to be lagged to the band with 1/2 galvanized lags or comparable structural exterior fasteners -
This gap is a huge deal, especially on large decks. Houses get badly damaged by direct attached decks transferring water to unshielded framing and sheathing.
reinstall the door and possibly use synthetic casing and paint it brown , or trim it with ripped down 5/4 treated deck boards and stain to match. If you drop the porch a similar trim piece can dress the bottom where this all would be exposed by the drop. Pvc or Trutrim (made from potash is great)
If the door frame has issues - and they usually do on the bottom legs as the end grain absorbs water just like it does when the tree is alive- you have to deal with that before you reinstall. You are good enough with your hands that between us all this can get done.
Always waterproof a structure like you aren’t going to do further work, and If possible make sure water can run off and evaporate - anything that keeps water out keeps it in- this is why non porous caulk accelerates the death of wood when it dams up water.
On a side note , new growth lumber is not as durable outside as the old stuff- not even close- so it takes a different approach today than it did decades ago to make a structure last. We have better materials in many ways, but build tighter and have to let water escape around wood of any species. Plain doug fir lumber out of oregon 60 years ago would outlast todays treated 2 to 1.