Cartridge-based games will save to the cartridge. A memory card is necessary only if you download DSi-ware games from the shop.
It doesn't 'need' a memory card (Memory Stick is a Sony card, the DSi uses SD cards), you can play games without using one. A memory card would be for storing lots of photographs, MP3s or DSi store games.
You can only put the memory card into the computer if you have a camera. You put the memory card into the camera then using the camera cord, plug the camera into the computer then put the photos onto the computer. Hopefully that was easy enough for you to understand.
Yes, the DSi has enough internal memory to store several DSiWare games at a time.
That's not how the DSi works - for one, code can't be executed from the SD card anyway. What you need to do is connect the DSi to the DSi store, and download the games there after you buy them.
you have to sign in
You can put a memory card in, for music and photos. But it does not come with one, nor is one needed to play games.
The DSi XL does not need (and cannot have) a RAM upgrade. It has a small amount of internal storage space. If you want to take a lot of photographs, store MP3's or download DSi store games, you will also need an SD memory card for it.
as many as your ad card can hold
You can download games/apps for the DSi via the DSi Shop.
You can download games/apps for the DSi via the DSi Shop.
The DSi's memory card is only for photographs and music. It cannot run code, or run anything on the memory card, it completely lacks the ability to. It is for storage only.