I can confirm that the default Windows Defender (MsMpEng.exe) is being used on Windows 10 when I have no third-party Anti-Virus (AV) installed.
Here’s an example where MsMpEng.exe runs when the Virus Scan button is clicked in GlassWire for the file Firefox.exe:
This issue has been discussed last year. I’m still not sure how I would confirm that the file is actually being scanned when using a third-party AV. At least, I haven’t yet managed to record an actual file scan using a third-party AV. I just tried again with BitDefender Free - I may try some others: