You’re (partially) right in that aspect. It’s calculated by your Garmin Device (watcch / headunit) by doing a round trip from Garmin Desktop App → Garmin Connect → Garmin Device → Garmin Connect
Below thread, there’s an article linked
But for VO2Max to get into intervals.icu, it’s a bit iffy unfortunately. It works for some, not for others and then if you do running, it can get mixed on what gets sent to intervals.icu