I don't know if this has been discussed here already, but how exactly does one become a Mechpilot? I know that you can only be born into being a pilot with the Clans, but how are things handled in the Inner Sphere?
You actually can be a clan mech pilot from outside the warrior caste, either by testing into the warrior caste, or being an IS pilot captured by the clan as a bondsman and being placed in the warrior caste. That said, it's rare to test in and extremely rare to get in via the isola route.
For the sphere, there's a bunch of different ways it can happen. The most common is probably just joining a house military or merc outfit and testing well enough to be assigned to a mech, same as how you'd get to be a fighter pilot or the like IRL. You could also inherit a mech from your family, but that's mostly a Noble thing. You could also just buy one and sign up with a merc group (and maybe a house army, not sure), generally they'll let you in if you're even vaguely competent.
There's a couple cases in the TROs where someone in a tank unit or other force has captured and salvaged an enemy mech and gotten to use it, but I imagine that's not common. Solaris VII teams might take promising fighters and train them as warriors, but if they do I think it's rare, Solaris is usual a BYOGR sorta place.