Yup you are completely!
Firstly it isn't just the launcher/homescreens which is touchwiz, but the entire UI is, you name it, the calendar, the notification bar, the system menus, all the icons are all styled differently to every other skin, but essentially they are all the same regarding their placements.
The same goes for sense, which is obviously used only on HTC mobiles. Same for sony, same for motorola etc.
However, since this is android, you can install 3rd party apps that can be used instead, they don't replace the stock launcher etc. The main thing about these 3rd party apps is choice as to how you want things to be, be it icons, colours of icons, number of homescreens, features like folders for apps (that is all mainly for a launcher) etc. etc.
The stock launcher will not use resources if you don't run it all and have a 3rd party one running instead, same for apps, if you don't use/open them, they won't use any processing power etc. they will only take up the internal storage space.
And some apps are also faster/smoother than others and the stock stuff.
And they don't really look the same at all, they are just similar, just like every other touch device that has icons etc.
Look through this thread and you can do the same on any android device pretty much.
http://forums.overclockers.co.uk/showthread.php?t=18137383&page=100