In short, no.

I would assume the port authority or controlling government agency has a different background check process than a airline. I would assume at the minimum FBI background check.

A SIDA check isn’t a secret at all. You can use a quick google search to see what will disqualify someone from having a SIDA badge. The TSA doesn’t care if you’re a burger flipper inside the terminal or a pilot, the same standard applies.

True, but holding a SIDA doesn’t mean you will pass an airlines background check, which I feel is the question being asked. I guess he has something in the closet that he is worried about?

For clarity, airlines will typically do two background checks.

One is the SIDA, which is required by law for 121 crew, period. Some crew get a SIDA badge at their domicile, but most do not these days. Probably the only places that still do that use the SIDA badge to open the gate to the parking lot, or some other logistical access reason. But you get the check regardless of whether or not you get a badge. And if you ever get convicted of a SIDA crime, you're unemployed.

The other is whatever additional HR checks they want (and are allowed by local law) to conduct. Something which is not on the SIDA list could be no-go here, ex, possession of kiddy porn, domestic violence, heroin possession. Some regionals today may only do the required SIDA check, ie they cannot afford to be picky... so you could have a felony and be an airline pilot as long as it's the right felony, and FAA aeromedical gave you a pass on your medical.

Do the majors actually care what regional you go to? Or do they just want 121 pic time?

They prefer SkyWest Mainline because you already have some Mainline experience coming in.

This. SkyWest E-Jets and the Mainlines basically line up with offers the moment you hit 1000 TPIC

No, it generally doesn't matter to the majors. Although if a regional has some sort of very contentious labor issue (ie alter ego operations), you should avoid that because those on the receiving end will hold a grudge for a long time and may eventually end up in a position to influence hiring. There are no such alter-ego regionals at this time, gojets was the last a number of years ago but at this point I doubt many folks would hold a grudge against a current gojets pilot.

For clarity, this is sarcasm.