[inferiorhuman]

  every time I look at how to use Caltrain from SFO I give up and
  rent a car instead.

BART really made a mess of transit to SFO, unfortunately. BART ridership never met projections so they played around quite a lot with service between Millbrae and SFO in effort to save money. For a while there was a Millbrae-SFO shuttle. For a while one line provided service during the day and one provided evening and weekend service. Even today only one of the two transbay lines that runs down the peninsula offers service to Millbrae and SFO.

Once you actually get to Millbrae you then get to deal with BART's whole NIH problem manifesting as a refusal (up until recently) to offer timed connections with Caltrain. And, of course, up until 2021 actually getting between the BART and Caltrain platforms involved a ton of walking.