iMac hardware

Hello people

I'm a little confused about the inside hardware part of iMacs.

So, I've seen iMacs with only a screen (with the hardware built-in to the screen), and I've also seen iMac's with ordinary cases (like a tower case of a PC). So, do iMac's come in those two forms?
Also, are iMac's meant to be opened inside? Like, on a PC when you want to make hardware changes (change motherboard, check fans, install harddisk etc.) you can easy open up the case and do what you want to do. Is that the case in iMacs or aren't they meant to be opened?
Btw, I'm not talking about a specific iMac, I'm talking about iMacs in general.

Thanks in advice Cheers !

 

Thanks for answering