Do i need to run my pc with the USB miner together or can it run standalone? I mean it need Internet or is it with wifi?
If it has wifi and it can run standalone can i usw a USB charger that supports 15w or do i need a USB hub?
If it works not standalone what do you use to run it? Can i use raspberry pi3 or raspberry zero w? I ask because of energy costs.
And for better understanding the GekkoScience Compac F can mine 300 - 500GH a RTX 3090 only 120 MH? Means 0.12GH?
You will need a computer, the Compac F doesn't have WIFI. You need a computer with cgminer installed on it.
It is difficult to say if the Compac F is the best usb miner, but yeah, it is an excellent one (a block was already found with one some months ago). You can have a look on Gekkoscience R909 too, if offers more hashrate. Gekkoscience products are stable and reliable, they are excellent.
You can also check the DIY project named Bitaxe, it runs standalone, and they are actually starting to sell the first batch (you can build them yourself or buy them already assembled). But we actually don't know if they are as stable as Gekkoscience's products.
For the USB hub related question, you should definitely use a strong USB hub with a Compac F, the USB port of a Raspberry Pi would probably don't offer enough power if you want to overclock your USB asic.