A SIM card is its own physical, minuscule logic board -- a tiny piece of plastic with a sparse amount of electronic circuitry on it.
An eSIM is a component soldered onto the logic board inside a smartphone.
Both do essentially the same thing, store your user account info with your chosen carrier. With a SIM card that info is added to the SIM card your carrier provided, and that's what you then inserted into to her phone. With eSIM, that same info is just typed directly into the eSIM chip that's already inside a phone.
An important factor is not all smartphones include support for eSIMs and not every carrier supports eSIMs. Her S20 does, but it also depends on if her carrier does too. But this is just so you're aware of what an eSIM is, she already has a SIM card and there's no needlessly switch over.
if the SIM card tray was bent in some way, that's not a good sign. Generally they're just plastic but not meant to be flexible so if it wasn't just flat and easily extendable that's not good. The little metal contacts on the edge of the SIM card need to match up directly with little metal contacts inside the phone, so something like a bent card tray will interfere with that. Thinking back, are you positively certain it wasn't flat?
Svim, I'm overwhelmed by your kindness, in explaining all that to me! With regard to the SIM card tray, yes, it had a very slightly curved appearance, and how I'm so sure of that is that, at first, the card would not stay where it had been placed — in its correct little rectangle, in the tray. (By the way, though the phone is not up-to-date, it was brand new and had not been opened, for the box was sealed.) When I attempted to close the drawer/tray, the SIM card kept flipping out. After I "bent it straight" (as it seemed to me, anyway), it stayed in place, and allowed the tray to be pushed in. Apart from the occasional dropped connections (mentioned earlier) with two relatives (both widely spaced in the UK), everything seems to be alright.
With regard to the difficulty my wife had in seeing the smaller details of photos, in her Files app, I set the View to "grid" instead of "list" as you suggested, and even my wife can now see what she needed to see. She is very pleased about that, and thanks you profusely for the tip, as I do.
S.