The rich text editor is available on all sites, and you can use the default composition mode setting to determine what your members will see when they open the composer for the first time. It’s set to rich text by default.
Members, though, can decide to use the composer toggle to switch back to Markdown mode. The composer will then remember this as their preferred mode, so it will reopen in Markdown mode until if / when they switch back to rich text.
The idea here is that we want to let members write in the composer that works best for them — admins know their communities and can make a reasonable choice about which default makes the most sense, but members should be able to choose a different mode if it works better for them.