You can record on your Garmin, record on Zwift and then merge the two files with Fit File Repair Tool.
In answer to the question above, the Garmin watch calculates this information with power, HRV and other factors that they don’t tell you. It gets calculated on the watch, not on GarminConnect.
Lastly, Physio TrueUp will sync the information back to the watch, but if there’s an activity there at the same time it won’t overwrite it.