The 20b covers 6 of these plants… totaling 240gwh or 3m vehicles (4m ID.3s).

They don't have to build absolutely every battery, they can still work with partners.

This is also a first investment. They can wait a year and invest double in a larger plant - when building something for mass manufacture you start with a smaller plant to learn how to do it, and then a larger one applying all the expensive lessons learned the hard way from the first. Sometimes this cycle is repeating dozens of times.

Won't a next factory cost less, due to the accumulated know-how, optimizations, improvements, etc?

I suppose the investment goes not just to civil engineering and land.