Who pays the caregiver? Does the pay come from the seniors social security?

Nope, not a penny for a husband's caregiving. Her SS is all hers.

Here is some information, hopefully it will answer your question. https://www.agingcare.com/articles/how-to-get-paid-for-being-a-caregiver-135476.htm

I have heard of parents paying a grown child to take care of them, as the grown child usually is giving up a paid job to be a caregiver. I've never heard of a husband wanting the wife to pay him to be her caregiver, but I am sure that does happen. In any case, one would need to create an employment contract, salary, hours, days one will work, what days the employee [the husband] would have off, etc. Plus the husband would need to pay payroll taxes.