If you didn't care about exposure time, you could build 2nm chips with brute-force electron beam lithography. But the limited throughput confines EBL to research and very low-volume applications. ASML's EUV-based processes are what permit industrial-level scaling, ultimately because parallel beams of electrons repel each other while parallel beams of photons don't.

I don't personally understand why suitable EUV light sources are so hard to build, but evidently, they are. It sounds like a big deal if China is catching up in that area.

They can do 7nm and 5nm. Multiple patterning basically. I don't know when it doesn't scale anymore. Moat likely 4x patterning is the max you want to do.