Welcome to Solus!
1) Yes. When packages are built during the week, problems are found and fixed, new packages added to the repo, etc., all the work is done in the unstable repository. At the end of the week (Friday or Saturday, depending on where you live), the team does a "sync," which is when unstable and stable become identical, at least for a short while. You need to be careful when you install anything from unstable during the week, because you could be doing that while somebody else is making changes that could affect you. More on that later ...
2) Solus is, by definition, a curated rolling release. That means that just by doing your weekly updates regularly, your 4.5 system will become a 4.6 system.
3) Sometimes the team will "cherry pick" a package from unstable to stable without waiting for the weekly sync. That's usually to fix a security issue, or a bad regression that's a showstopper untill it's been fixed and sync'd. Then, of course, it's no longer just in unstable, and if it's a package that matters to you, all you need to do is reinstall it.
Installing a package from unstable while you're otherwise based on the stable repository is quite easy. You just use an eopkg install command that targets that package from that repository (based on its URL). Creating that eopkg command can be confusing for a newer user, so the best idea is to ask here in the forum for someone more experienced with solus and eopkg to advise you. It's a very friendly place! Then, just start a terminal, paste the command, and execute it.
One gotcha you should understand though: if you change to the unstable repository and do an update there, the only time that unstable and stable both contain exactly the same contents is imediately after a sync is done. After a little while, all bets are off, as work there continues, especially if you do an update sometime later during the week. And having done that, if you change back to the stable repository (Shannon), the unstable packages and their dependencies from unstable are not automatically reverted to their equivalents from the stable repository. If you cause any problems during that change and change back, it can tough to find and fix them manually.
The best advice is generally to stick with the stable repository unless/until you have a very clear understanding of how it all works, and you have a serious reason to change to unstable. Otherwise just be patient until the sync that happens at the end of most weeks.
