Do LAN transfers use both Ethernet and WiFi by default? So, I'm doing a 100+GB transfer over my LAN from my iMac to my NAS, I was simply wondering if it would utilize both the ethernet and the wifi for the transfer. If not, is there a way to enable transfers to use both?
Do LAN transfers use both Ethernet and WiFi by default?


No.  The default action is to use the one with the highest priority.  This is usually done by the order of the interfaces that you specify in Network Preferences.  Following that, the next order of priority is network latency.


  If not, is there a way to enable transfers to use both?


What you are referring to is called link aggregation or bonding.  Your Mac is definitely capable of doing this, however, you must connect to a switch (usually a "smart switch") that also has this capability.  So, unless your switch has this ability, you can't do it.

Also, you can't bond WiFi and Ethernet; link aggregation is for bonding Ethernet links.