I will store it in my garage, in a shelve among other things. It is quite dusty, so I’m thinking of building a plexiglass hermetic box to keep it while powered off. Would it be a problem to keep it closed also while printing? This would change the type of box I’ll build, because there is not much space and I’m trying to save the most of it

This is a normal thing to do, actually. most printers benefit greatly from being printed in an enclosure.

How do I store the filament? I (currently) have only one filament (black PLA), so I see no need to remove it from the printer each time, but leaving it “connected” (I don’t know how to say it) will not allow me to store it in a different way the printer is stored. Do I need to store it in special ways or can I leave it connected? (And bonus question, what is the correct word to say it?)

It depends on a lot of things. For PLA, I've found ziplocks + the moisture packs they come with to be sufficient protection. If you want a dedicated filament storage, a large rubermaid (or whatever) bin with loose silica gel is going to do just fine as well.

There are also plenty of STL versions for silica gel "containers" that fit in the hub of a spool.

For other filaments, it depends greatly on the type of plastic. PETG needs to be kept dry. You can dry any filament in a food dehydrator if it doesn't get too hot. (PLA, don't go above 55. 45 is better... and keep it slower. 45c it might take 4-6 hours.) PETG can be dried happily at 55c, in the same amount of time. Or, if you don't mind playing the waiting game, you can stuff it in the bin with silica gel... and just let the gel do it's thing. (the one linked above can be recharged in an oven or food dehydrator. I'd recomend getting something cheap explicitly for drying filament and recharging the gel at the same time. I use a presto food dehydrator... and I got some extra trays (which promptly removed the mesh from every-other tray, so I can stack multiple reels.) The presto goes down to 90F or 32C,

If I don’t move the printer, how often should I calibrate it?

Define "calibration"... eventually you'll learn what needs to be checked and how often. Generally, i check belts and for anything obviously wrong every fresh session of printing. but I'm not going to be tweaking it everytime. it's just a good habit to check your printer over every so often.

I use UBL and mesh bed leveling, and find I can go about 2 weeks of constant printing before I need to pull a new mesh. IIRC, the ender 3's don't come with a bed probe; so you're going to be manually leveling your bed; which should happen every new session or so. You can reduce it some by getting silicone bed supports (instead of the springs.).

Things like esteps / mm, are only calibrated if I change something that would affect it (For example, a new extruder, or a change to the mechanical belts, or a new stepper driver/motor.) Or if I'm chasing something being waaayyyy off.