How you upgrade bikes depends on the type of bike - road/gravel bike upgrades are based on distance ridden; TT bikes on duration; and climbing bikes and MTBs on elevation gain.
Bikes are also categorised into entry level, mid range, high end and Halo. The distance/duration/elevation you need to ride, as well as the drops cost for upgrades and what those upgrades at each level actually are, all depend on which of these categories the specific bike frame is in (lower end bikes can be upgraded quicker than higher end basically).
This post on Zwift Insider breaks it all down in detail https://zwiftinsider.com/bike-upgrade-details/
Bikes are also categorised into entry level, mid range, high end and Halo. The distance/duration/elevation you need to ride, as well as the drops cost for upgrades and what those upgrades at each level actually are, all depend on which of these categories the specific bike frame is in (lower end bikes can be upgraded quicker than higher end basically).
This post on Zwift Insider breaks it all down in detail https://zwiftinsider.com/bike-upgrade-details/
