What happens to the old area from where the hair is taken?

It's taken in small strips from the back and sides of the scalp which, for whatever reasons, aren't prone to the mechanisms causing male pattern baldness on the top of the head and hairline. So theoretically those areas where the hair was mined will now be thinner, but they do it in a manner that you'll never notice the decrease in hair in those areas.

I heard from a barber, that despite claims hairs from the back of the head are different follicles, and won't die off, this is all lies and marketing bs.

So you need endless treatments, or else take testosterone blockers.

Well

1. even back of head hair thins eventually in balding people, but generally many decades later than the front or crown. By the time you're losing that hair, you're probably a lot older and care much less

2. taking finasteride indefinitely is supposed to drastically slow or stop further thinning or loss. Of course as in point 1, you're probably going to lose that battle eventually, but again you'll be much much older when the damage gets serious than you would have been otherwise

And you continue to recede so you will need follow-up treatments for that. If you're too far gone it's just not practical.

It's a meme at this point, but for most people shaving their head is the right option.