How do people get around Paris when transit employees don't feel like working that day?

There are multiple route to get from point A to B. For example, from Versailles to Invalides, you can take: RER C then Metro 8. Or TER N then i can choose from two different metro line at Montparnasse Or TER U then RER A.

There is also buses, bus since the rail is faster, I never take it.

In case of strike the network is never fully down, people that can remote work do it, so there is a lot less of people transiting. On the biggest strike you can loose an hour or two while commuting, for small strike, it will get more crowded.

They don't, because that would be the whole point of a total shutdown in a coordinated, all-modes transit employees strike. Ask people in London, they have that on a semi-regular basis.

Otherwise, there is no such thing as "transit employees not feeling like working" - thanks to the magic of economy holding a metaphorical gun to the heads of most people. You work whether you feel like it or not.