If you really care...
Here is a link that explains the difference between the file formats:
https://www.guidingtech.com/9661/dif...ebook-formats/
Basically....Amazon bought the original 'mobi' format in 2005....the format was discontinued in 2011...but Amazon's 'AZW' format is based on that original mobi structure. That's why you will find some people referring to the Amazon AZW as the 'mobi' format. Even though Amazon has moved on since then and have updated their formats, AZW3 (and AZW4), the AZW format is just used for backwards compatibility for those older devices - which, as mentioned above, are still out there. There is still some speculation that Amazon may actually start to openly embrace the ePub world
soon...I'm not holding my breath on that one...and pretty much every other publishing platform out there will natively read the ePub format.
The good news is that it is really easy to convert a well made (and open source) ePub into those proprietary formats. I would even say that most professional ebook makers start with a good, clean, ePub as their master document, then add any tweeks/changes to deal with the requirements of the lesser formats.
Cheers,