If I pay for the 50 GB do I get 50 GB each month

Your monthly fee is to maintain access to a single 50GB slice of storage. To get more than 50GB you’d have to pay for a larger storage subscription.

You get 59GB of total storage. You pay monthly but the space does not increase. It is similar to renting physical storage space or renting an apartment -- you pay every month but the apartment doesn't get bigger.