Are you using the RS Krakow, Warsaw & Gdansk guidebook to help plan your trip? I would highly recommend it - get an additional guidebook if you intend to visit places in Poland that RS doesn't cover. Note that Auschwitz is well covered in a separate chapter following Krakow.
If you intend to arrange your own transportation, the steps documented by qq above should be all you need to purchase tickets for individuals to visit and join a mandatory guided tour. For public transportation, information is available on the official site, although you have to dig for it: https://www.auschwitz.org/en/links/information-for-visitors/
RS also lists several drivers that will provide transportation from Krakow to the site. However, you must make your own arrangements [purchase tickets] to join an official tour. A web search would most likely come up with more options for drivers.
I was there on 4/15/24. I chose to purchase a guided tour package from Krakow - from Discover Cracow. Transportation was via a 30-person Mercedes extended van with 2+1 seating. It picked us up in front of Krakow Glowny - there may also be options for pickup from some hotels. The advantage of a packaged tour is convenience - they get you there, have your entrance ticket ready for the tour, take you to Birkenau, then take you back to Krakow. It was coordinated with the official Auschwitz docent, who was excellent. Note that the bus ride is 90 minutes each way - the route is entirely on secondary roads.
RS mentions these additional firms offering packaged tours: See Krakow, Krakow Booking & Cracow City Tours. There are most likely others.
I think that if I don't want one of their tours, I wait until late in the day which I don't want to do...but there is nothing that I find that clearly explains that.
I think they make it pretty clear - an official tour is mandatory for entries before 16:00. On the same page offering tours for individuals for the various timeslots for a given day, you will find "Tour for individuals without an educator" starting at 16:00 - those tickets are free, but a reservation is still required - and tickets are limited for each timeslot. So if you want to visit on your own without a tour, find your day, Visit for Individuals, then scroll all the way down to the timeslots for late in the day.
Note that you will miss the excellent narration provided by the official docent - in our case she was incredibly knowledgeable and greatly added to the experience. RS offers a detailed walking tour in the guidebook, but then you will still have to navigate between the various buildings. So unless you have the details memorized, you may have to stop and refer to the guidebook quite often to determine what you're looking at, where to go next, etc.
Birkenau is easier because of its vast size - you won't encounter the crowding you'll experience at Auschwitz I. But the scale is the real message - seeing the infrastructure that was in place to execute 8,000 people at the same time. We were told that we'd walk three kilometers for this part of the visit.