Does the whole line download?

If I download Fortnite on a 50K line now, is the line very slow?

Depends on what the server delivers to you. If the full bandwidth can be used, then you hardly have any reserve for other things on other PCs or cell phones.

It depends on how much the launcher (or server) allows and can or the console. Of course the line will slow down but it won't pull your whole house away from the internet. The line will always pull what it can.

Depending on what connection your PC has, then yes. My parents always complain because my PC uses up all the data in our network.

It can happen that the download takes almost all of your performance and other things then run relatively slowly. However, you can limit the maximum speed that the Epic GamesLauncher uses using Netlimiter. https://de.phhsnews.com/how-to-limit-any-application-s-bandwidth-on-windows6013