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

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.

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.