The eUICC is the chip embedded in the device. eSIM, in everyday use, refers to the service or the profile downloaded onto that chip. Put simply, the eUICC is hardware your phone already has, and what you buy is the profile that goes into it.

It depends on the manufacturer and the model; there is no single figure that applies to every device. The reliable way to know is to open the SIM menu on your own phone, look at the entries already there, and note how many it will let you keep active at once, which is usually fewer than it can store.

Usually not, because a great many codes are issued for one installation only. Some providers can reissue under certain conditions, but not all of them do. If you have already deleted it, contact support with your order number as soon as you can so your case can be checked.

No. If the phone supports eSIM, the chip was fitted at the factory. There is no part to replace and nothing to open up. The whole installation happens through the settings menu over an internet connection.

Generally yes. A factory reset usually deletes profiles along with everything else. Some systems offer to keep them or ask for confirmation first, but do not rely on that. If you still have an allowance left and a reset is coming, ask the provider beforehand whether reinstalling is possible.