How does a Mac choose between Wi-Fi and Ethernet to work with? My Mac is connected via a 1GBit Ethernet. However, from time to time I do turn on WiFi and let it connect, typically because I need to use AirDrop etc.

When I'm done with AirDrop I usually turn the Wi-Fi off and stay on Ethernet. 

My thinking is that "wifi is slower" and my Mac may use it when downloading or uploading instead of my fast Ethernet. Now I realize this is not based on anything else other than fear.

Is there truth to my fear? 

If a Mac has two active connections, Wi-fi, and Ethernet -- what will it choose to use when I'm uploading a large file? Is there a possibility it would choose to use the slower Wifi connection? Or does it employ some kind of algorithm to figure out which one to use based on availability, type and speed?
It's possible to set network access interface priority order. Go to System Preferences → Network, click on the lock icon to unlock the preference pane. Now click on the gear icon shown towards the bottom of the network interface list, and select Set Service Order... command.



Now, in the new pane that opens, you can drag the priority network interface (Ethernet in your case) to top.



(I do not have a LAN interface, so the same isn't visible in the screenshot).