Amazon/Kindle and ePub (everybody else) are the two major formats.
For Kindle formatting, you can send Amazon an epub and they will convert it for you. See
Kindle Direct Publishing for more information. For do it yourself, you can use calibre to convert your epub to mobi (for antique Kindles), AZW3 and with the help of the KFX Output plugin to KFX. For quick testing, you can use Kindle Previewer on your computer though it is not as useful as it once was.
One recommendation is to test your epub and Kindle content on multiple apps and devices. For my uses, I use a couple of versions of ADE and Thorium on my Windows PC, Kobo, Pocketbook and Kindle apps on my iPhone/iPad plus physical Kobo and Kindle ereaders.
I don't bother with worrying about Android apps since they tend to be very cavalier in their treatment of my carefully crafted CSS and—generally—their epub3 support is pathetic.