Not all rolling stock assets are set to carry goods (including passengers in the case of carriages). The quick way to check is, as PWeiser suggest above, is to turn on the Commodity Overlay in Surveyor. This is in the Main Menu -> Display (the 4th icon) -> Commodity Overlay -> Always Visible. A green bar above each carriage will indicate that it is passenger enabled. The bar may be fully green (indicating full), part green and part grey (partly filled) or completely grey (empty). Being passenger enabled does not always mean that "passengers" will appear inside the carriages - that is a choice made by the asset creator.
With the Commodity Overlay switched on, each station should (if it is passenger enabled) also display a green/grey bar indicating that passengers are waiting.
I have never bothered with point (2) listed in the post above - it seems to have no effect on the loading/unloading of passengers.
"Drive To", "Load" and "Unload" commands are essential when using the AI to do all the work. Otherwise if driving manually then simply stopping at the station will automatically perform the load/unload tasks.
Development: Trainz Plus - b119450
Testing: Trainz Plus beta - b119450
GTX1050Ti, RTX3060Ti and RTX3050Ti, Win 10 & 11