Does MacOS "load-balance" TCP when more than one interface is available? If an iMac has both cat-5 and WiFi connections to the world, will MacOS use them both to optimize the overall speed?  Or pick only the fastest for everything (since the ISP side of the modem is slower than Cat-5 anyway).

If yes, in what older version (if any) was this not true?

This is NOT a duplicate of any question that has two ISPs.

All devices are connected WiFi to an AirPort Extreme.  One of them is also connected by Cat-5.  I am not currently using USB or Thunderbolt or FireWire.  The Cat-5 obviously has a different IP address than the WiFi.  The Airport goes through a Motorola DOCSIS to the world.  Ookla says the wired connection downloads at 79+ MBps and the WiFi at 50+ MBps.

Also, I wasn't thinking so much of link aggregation, but like letting one app use the wire and another WiFi, giving each app more bandwidth than if they had to shared the same interface hardware.
No, macOS assigns one route to a network. You’ll need to set up link aggregation which does extremely effective splitting if you can’t have a router that selects egress efficiently and you route all your traffic to that router. 


https://en.wikipedia.org/wiki/Link_aggregation