SIM cards have 3 sizes but the "chip" is the same size, so they all come as "standard/mini" size, then have break away rings to change the size to "micro" or "nano" size
Seen here:
https://cdn.3g.co.uk/userfiles/guides/e/SIM-Card-Size.jpg?v=0.1
Example of what service providers SIM may look like when you get it:
https://images-na.ssl-images-amazon.com/images/I/811h18YTYOL.jpg
You can see the break away parts, to make the 3 sizes
Keep the rings if using nano size, as your next phone may use micro, the rings are just to position/center SIM "chip" in the phone makers slot/holder
I think I read that the Cat 42 had dual SIM slots, this is mainly used for international travel, so you can have your "home" SIM provider card and then have a "pay as you go" international SIM card so you are not paying Roaming fees when abroad
OR
If you want two phone numbers on the same phone.............................for whatever reason
Some cell phones now come with eSIM cards, a built in SIM, not removable, and also have a SIM slot so are dual SIM capable
eSIMs used to be a bit of a pain to get activated, especially in iPhones(what's new, lol), but its getting better
And changing providers on eSIM can be............................well, a long process, since you have to get BOTH the old provider to release it and the new provider to activate it................................if you like being on hold this is a process you will love