Not sure about 1, you can probably just google search it
For 2 though, it depends. If you bought a SD card for your roms, then you have to go with cia, if you have a microSD though, you can use an adapter for cia too, or you can go with 3dz. C
ia's are more of a hassle, though I prefer them because there is a less chance of a ban. Noone has actually gotten a ban, but since cia's don't need headers, a ban is less likely. Other difference is cias are loaded from your home menu, instead of the gateway thing.
You convert 3ds to 3dz by using instructions here: https://gbatemp.net/threads/how-to-use-scene-dumps-with-gw-2-2-online.365829/
I thought that this tuto is better : https://gbatemp.net/threads/scene-roms-online-tutorial-for-dummies.370589/
Am I wrong ?
Can I use a header of an another ROM which has one to put it in one which doesn't have?
Yes, you can use the header of another rom. You can choose from a dump of a cart you own or publicly available headers.Can I use a header of an another ROM which has one to put it in one which doesn't have?
Yes, just download a header for any game from anywhere, then modify it according to this post:
https://gbatemp.net/threads/how-to-use-scene-dumps-with-gw-2-2-online.365829/page-4#post-4991676
Once you have a modified .bin file for your game, patch it with your .3ds rom with Gateway's ROM patcher. Rename .3ds to .3dz and you should be able to play online.
I use these instructions and i was able to play mario kart 7 online.
Oh one more thing, make sure you have the latest updates for your games too (ex update 1.0.4 for smash 4)
Yes, you can use the header of another rom. You can choose from a dump of a cart you own or publicly available headers.
I will need to have the latest update for my games so I will have to update them using the eSHOP on EmuNAND?
And how can I know the Manufacturer (1.) and the "Unknown" (3.) ?
Yes, it refers to the manufacturer of the flash memory on the original game cart. It has nothing to do with your SD or micro SD card."Sandisk" should not be the maker of your SD, but the original cartridge maker, isn't it ?