I've worked both the commercial and corporate sides of this business. Long term employment is really pot-luck chance. There are plenty of folks who've worked for the same airline for their whole career. Then there's others that have jumped from a dozen or so.
Longevity is really based on the company's financials. On the corporate side, the Coke's, Dow's, Exxon's and any other multinational's have been around for a long time. Executives really like their aircraft, so the aviation department is usually one of the last to go if business turns sour. At the same time though, if you take care of a rich family's aircraft and Daddy dies..... so does your job.
There are numerous types of corporate aircraft. Some are a major pain-in-the-ass to work on. Some not so bad. Given the opportunity to work on Gulfstreams, Falcons, or the larger Cessnas... I'd take that over Hawkers, Lears, and King-Air's. You can't be picky though, a job is a job.
On the commercial side. Company financials rule also. Yes, American, Delta, and United were everyone's dream job up until this plandemic..... now nothing is for certain. Cargo is still going strong. Limited flight benefits on the cargo side vs the airlines. Then you have to decide on if you want to be union or not. FedEx isn't, UPS is. Some extra BS to put up with on the union side, but there are pluses to them also. UPS is highest paid in the industry.
I did Continental for 4 after the Air Force, Conoco for 5 years on the corporate side, and FedEx for 20 after that. I can do both sides (never done piston-poppers), but I prefer the big boys. Being a freighter dog does keep you away from shitters, seats, and passenger convenience items.
Whatever you do..... make yourself valuable. Avionics/Electrical and composites are hot items. Being a damn good well rounded mechanic who understands systems and can troubleshoot them is "always" a winner.
I'm sure you've heard the term "licensed to learn". No one expects a fresh A&P to step in and be self sufficient right off the bat. You become competent over time, but you "never" stop learning
Things like daily check items, tires, brakes, and fluid servicing come pretty quick. Others like component replacements, rigging, and troubleshooting are not required on a daily basis, so skill sets take longer to obtain.
Most major airlines have some sort of apprenticeship programs. Maybe some FBO's also. If you end up working corporate or piston-poppers, you'll probably find experienced mechanics who will be more than happy to teach you.
It's in everyone's best interest to pass along knowledge to the next generation coming in.
I was a "young buck" once. The old farts taught me.
Guess what???? I'm the old fart now. This site was built to pass along whatever I have learned to others.
The whole point of a capacitive fuel quantity system using vertical probes throughout the tank is that capacitance in parallel is "additive". If the aircraft is in a bank , the down-wing tank inner probes might have less fuel on them (less capacitance), but the outer probes will have more fuel (higher capacitance). The overall capacitance output remains the same. Thus, the quantity indication remains the same. The high wing would have less on the outer probes, but more on the inner probes..... the same rules apply.