Does that mean i use a jtag to infect my normal account with for example (xp lobby)or mod menu or something like that

TTG-DaBoMb wrote Key Vaults or kvs no longer work to get a jtag online. You can not get a jtag online and may never be able to again. If a way is ever figured out, it will probably not be using new kvs.

The other guys answer was completely wrong

you are wrong as well jtag's can and are online

It is a file on the xbox that Xbox identifies when it goes online to make sure your console is not banned from xbox live. Before the dashboard update (the 3rd xbox dashboards version) jtags would extract these from broken xbox'es transfer them to there xbox, and get online for a couple hours. They would then get banned and then to get on, would need to buy another. Keyvaults were a typer of currency and many modders still have quite a few in case a bypass is found, some (Under 15 people) have a bypass to get online but will not release in fear of all the games being ruined by mods.