Once an edition's ebook is built, Preview next to its download opens it in the browser. Turn the pages with the arrows, the arrow keys or a click on the page's left or right half, and jump to any chapter from Contents.
Kindle, Apple Books and Kobo each show the book on their own page size, with their margins, their usual font and their way of aligning text. Try the White, Sepia and Night themes and a few text sizes: readers change both, and the book should read well with any of them. Japanese editions show in vertical lines, turning from right to left.
The preview is close to what the reading apps show, not the same: each app uses its own fonts and may change details. Before you publish, also open the file in the store's own previewer, such as Kindle Previewer for KDP.