I am not a smartphone technician nor a software tech. I am a travel enthusiast who needed updating re unlocked travel smartphone capabilities (at reasonably low cost). I have researched & am happy to share my resulting layperson understandings (trying here to write a short form primer). Perhaps my thoughts will save you some time & point you in clearer directions for your own research & decisions regarding a good travel smartphone setup.
Travel to another country, take your smartphone… Want local service on this phone @ low cost? (= the challenge. Let’s avoid the dreaded, expensive roaming charges & other costs). There’s been lots of progress lately (mostly in the last 2 years?).
For many years (& still possible), many have been taking an unlocked phone with a SIM card slot in the phone when traveling. By buying a new plastic SIM card in your destination country (preferably before leaving the arrival airport, I hear) & swapping this SIM card for your home phone service SIM card, one might gain a new local no. in the destination country & local, lower cost rates. One chooses the new SIM card for a certain amount of phone time/data. One traveling to a lot to different countries might accumulate lots of little plastic SIM cards to look after (one for France, one for England, etc.). They need careful storage; e.g. don’t get dust on one & put the dusty card inside your smartphone….). Put the home phone no. SIM card back in when @ home. If you have a newer phone with 2 SIM card slots, can keep 2 SIM cards in & both phone nos. will function. A useful & less expensive travel system; not perfect. Sometimes these SIM cards have been vulnerable to scammers in various ways.
Perhaps worth mentioning here that I have downloaded virus protection security to my smartphone.
Next Evolution: eSIM chips have been embedded in some newer smartphones when they are made. One takes out an internet account with a provider, purchases & downloads software code to the smartphone = the time/data you purchased for the destination country. Rumored to be a step up in security, & no loose little plastic SIM cards to babysit.
Also Evolution: I have now seen 2 companies based in Germany who make/sell SIM cards with an embedded eSIM chip. Insert one of these physical SIM cards & gain eSIM capabilities. I might try one of these out because my newer smartphone does not have an embedded eSIM chip.
And Now: (e.g. for Samsung phones I think the rollout started ~February this year, for their newer flagship phones): one can download a software update, & if your phone is capable (e.g.if your smartphone has an EID serial no.), you might have gained eSIM capabilities to use an online eSIM account. No physical eSIM chip needed if a software update might do the trick.
That’s about where we are; options that I have read about so far. Progress! I think, for the traveler wanting low cost local phone time/data @ their travel destination.