Confused (or just thick?)

I’ve been moving files around using a portable drive, in particular to archive a whole bunch of iPhone & iPad video files from a number of different iPhones and iPads (different vintages and owners) to an older Mac running Sierra.
I’m confused in that some of these files are .mov and some are .MOV.
I’m even more confused in that some of these files were created sequentially on the same iPhone within minutes of each other and only the .mov files will play on the receiving Mac.
OK. scrub that. After a bit more delving to see if I could isolate any of the source devices, I’m finding that some .mov files will play and some won’t, and also, some .MOV files will play and some won’t.
Another confusion arises with a number of files being duplicated with the addition of a 2.mov or MOV and neither will play. But then there are some like that where both will play, and the odd one or two where only one will play.
It could be my age.
The only thing I can relate all this to is that some of the files were delivered via Airdrop, some have been delivered through iMessage and others (not many) by eMail.
At the moment I’m just looking at the files the Finder while I work out how best to group/catalogue them.
I guess when it comes to creating the archive I will have to convert all the different formats into a common one. With that in mind (and a new Mac Mini in the offing), I would appreciate opinions as to the best options/software for doing that.
Thanks