Will Mac Migration Assistant automatically choose Ethernet when Ethernet & WiFi are both connected?

I'd STRONGLY suggest forgetting about using Ethernet and use the Time Machine backup from the old computer and connect it via USB cable to the new computer. Why, it's simple it's the most reliable method! If you use a bootable clone rather than Time Machine than connect that via USB.

Because the machine chooses the first available network the list's order. If you want Ethernet then you must move Ethernet to the top of the list's order displayed in Network preferences.

If both are active on your computer then turn Wi-Fi off temporarily until you finish the download.

See in System Preferences->Network which of the two is on top and has a green dot. That is the one that is active even if both are turned on.

Also I second the suggestion to use a directly attached drive instead of Ethernet if possible.

If you are migrating from a mac that has Thunderbolt then use Target Disk Mode for the fastest possible migration-especially if the drive in the older mac is an SSD.

No. It will choose the currently active connection. If you want Ethernet then turn off Wi-Fi and select Ethernet.