It's a happy feeling to get it going, huh?!
When you run something in a module, it doesn't get loaded. It just runs in place, in the ROM.
As far as registers go, there's a limited amount of RAM, and it is split between program memory and data registers. Nothing happens to the RAM, but system variables hold a position for that wall so you don't for example accidentally write data over part of a program if you didn't have SIZE set for enough data registers.
As far as age goes, these things seem to be very well made, and holding up well. The original version, which HP officially internally named the "Coconut" (not the "Fullnut") before they had a model number for it, was less reliable but apparently easier to repair. In 1985 or '6, it was replaced by the "Halfnut," named that because the effort was to reduce costs, and early ideas included half the number of ICs and half the number of ports. The 4 ports remained though, fortunately.
My 41cx from '86 is a Halfnut, and I have had only very minor problems with it, like that two or three of the keys have to be pressed harder than the rest in order to register, and when I've dropped it, I've had to bang on the side with the heel of my hand to restore the connections to the bridge board to get it to go back on, then set the time and date again. The memory is not lost though, and I have not had a MEMORY LOST condition in three decades. When I did, it was because I was new to synthetic programming which gives you access to more registers and lets you make programs more efficient, get more frequencies and durations of tones, and other cool stuff, but may result in wiping out your memory if you don't know what you're doing and you do something wrong and it can't make sense of the program chain anymore so it starts over. That never happened again after I got the ZENROM when it was on sale, which makes all the synthetics as if they were standard, like STO M.
The 41 is still going strong in a sense though, with very smart engineers still coming up with new modules for it. Instead of plugging in a lot of hardware modules, we get something like Diego's Clonix module and put a lot of ROM module images in its flash. Angel has been quite a prolific writer of new modules, with new functions written in MCode for maximum performance. See my HP-41 links in my links page linked in my signature line. The Systemyde page linked there has a lot of his modules' manuals. My own CAT2 now shows this on the monitor with the HP92198 HP-IL 80-column interface:
The 41
CL transplant board for the Coconut 41's (which I don't have) is more-modern hardware that's up to 50 times as fast as the original (hence the "L," the Roman numeral for 50) and comes with over 300 modules already installed. Then there's SwissMicros and their DM41x which is even faster but does not have HP-IL.
So welcome to a world that is bigger than you thought, and has advanced a lot even in the last five years!
How I got into it and what I've done with it is told at
http://wilsonminesco.com/HP41intro.html .