> So if you're a Texas resident, the CA company would have had to register in Texas to have you on payroll, and that relationship would be governed by Texas law.

I don’t think this is fully correct. My understanding is that it’s about where the work is being done, not the residency.

For example, many businesses in Portland, OR employ residents of Vancouver, WA (right across the river). When COVID hit and those WA residents became remote employees, many Portland businesses had to become registered in WA because that is now where they had employees located.