You could connect the cell phone to the tv via bluetooth, can you do that with a screen for a computer?
So I just wanted to have something with which I could look a little bigger on YouTube videos or on the cell phone, is that only possible with a screen or do you need a tablet / laptop?
You would need a device like a laptop or a PC.
If your computer has no Bluetooth screens, it will not work.
Can't you just do it on a PC?
The cell phone and the PC must support DLNA and of course it is only possible via Wi-Fi, Bluetooth only sends and receives small packets which does not work for that