I used to use a Mac with some Steam games like CS:GO, I'm gonna build my gaming PC soon (on Windows 10). Is transferring Steam games possible to another computer with another operating system possible. And if, how? Do I just log in and install the games or is the process different?
If the games have Steamplay, then yes, games that you have on your Mac will be able to be played on your Windows machine.
If they do not have Steamplay, you will have to buy the Windows version even if you already have the Mac version.
From the Steamplay Documentation:
Steam Play allows you to purchase your games once and play anywhere. Whether you have purchased your Steam Play enabled game on a Mac or PC (both Windows and Linux), you will be able to play on the other platform free of charge.
You can copy over your Steam folder (by default C:\Program Files\Steam) from one Windows computer to another and as long as you copy over everything and you don't change any paths, and then you re-install Steam on the new computer with the same paths as you had on the old one (and yes, you will need to re-install the Steam client), you'll have minimal work that needs to be done on the new computer.
Paths can be altered as well, but then things get more complicated -- keeping paths the same is the easiest way to make this work.
You can also just copy over the Steam steamapps directory without the rest of the Steam client and as long as the Steam client on the new computer knows of that directory, it won't have to download games again on the new computer, though you will have to "install" each one, but in that case "install" will mean "verify that everything is valid, and maybe do some minor setup work" rather than "download all 50 GB of stuff again".
If by "different operating system" you don't mean Windows 7 -> Windows 10, but instead "Windows 7 -> Mac or Linux", then this isn't going to work, but do be aware that in the vast majority of cases, if a game on Steam is available on Windows and also on Mac or Linux, if you bought one version you'll have access to the other OS versions as well for free.
That said, there are a few exceptions -- "Call of Duty Black Ops" and "Call of Duty Black Ops Mac Edition" comes to mind.