I shared only a small bit on another reply, but to give more detail - the tray app and installer put your data in one location, while running the executable/portable version directly will do this in another spot.
This is for a few reasons, but the most clear is that there's an unintentional bug that became a feature. Jellyfin as a console C# app was built to use standard folders for data storage (the XDG defaults). There are variables on all operating systems that make these correspond to different folders. On Windows, some of those happen to be in AppData. This was how it worked from early on, until someone mentioned that it should be ProgramData instead. For the installer, we had a chance to "right the ship". We decided a few things - we would detect where your data was, and use that going forward. If it was a new install, you get an option to choose a location, but it would default to ProgramData. More on that in a second.
Regarding the testing - it was felt that not providing an installer was the safest choice here. 10.7, even in a release candidate, is a mostly one-way trip. I'd have to check to be sure of the exact details, but for a few current versions, we have done database migrations in launching a new version of the server, and I believe Items (again, have to check) is what moves in this one. We usually have a way to go back. We don't delete the original files, but we add on different things to the ones we're using. Just to be safe, we always ask that people make a backup (which should be clearer).
If you download the manual release here, without installer or tray app, then when you launch it, it should, for most users (anyone in the past 18 months I believe), start a separate data folder. Under the console app/a Windows interpretation of the XDG data folder. The his means that you don't accidentally hose your install, and that you can still test if you want to.
The installer code is unchanged at the moment. It's a fancy file extractor at the moment, with some detection for writing a path to the registry, including a shortcut. The tray app has indeed had minor changes, but we've been testing them in development builds for a few months now. Though most of the development team runs Jellyfin on Linux, that is not exclusively where it is tested. I'm on the project lead team and I maintain the Windows installer/tray app, and the macOS app wrapper, as well as handle publishing the client apps to all the stores (Roku, Google Play, Amazon, Microsoft, Apple). I also have multiple Linux servers at home (including where my installation runs), as well as several macOS, and Windows machines, in various imaged states. Some have existing Jellyfin installs to test migration installs, some are "blank" to test new installs, and so on. I'm also not the only one on the team that uses platforms other than Linux.
We're also dealing with some other issues at the moment (plugin system, etc). The majority of the things that need testing are in the server code and web interface, not on the other side. I had contemplated making a manual build of the RC with installer and tray for Windows, and the .app for macOS, but I also wasn't expecting to do that right away on the 31st. I would have hoped to have a day (but that's just me). I think we're going to do an RC3, at which point I'll try to get that ready.