Sounds like the DM has to buy/own everything

Trying to decide which service to use to move my game to virtual and have a question.  The way the Subscription reads the compendium is only shared if you own the game.  I take that to mean that the dungeon master has to own all the materials for the game and it doesn't matter if the players own anything.  In other words, with other platforms if one player buys the D&D Volo's guide and joins a game then all the players in that game can play as goblins and the DM can use monsters from that book as well.  From what I understand here the players can't contribute to game content and the DM must purchase it all. Am I understanding this correctly?

As I am recently migrating online due to the virus, I was interested in this also. 

Pretty much yes, however, players can gift GMs resources. Additionally, Roll20 has nothing that prevents you from manually entering data on the sheet (sort of like you would on a paper sheet).

None of the games I run have compendiums or any other flash-bang stuff. As Scott says, it is all doable depending on how much work you want to put into it. I personally have lots of handouts, character sheets, images, sounds, etc tied to my games while a game I played in last month had one graphic and folks only used Roll20 to roll dice. Your mileage may vary.

I understand what you're asking (yes there are some features that if the GM owns, everyone gets access to it). You and your GM and friends can, honestly, play this Roll20 website without buying anything. No one buys anything. Just use the free account the first time. Have whoever is going to GM press a free button called "Create New Game", then get the invite link and share with each other, then everyone press LAUNCH GAME and enter the blank, free, empty tabletop. Go from there. Best starting point.

Also note that it is technically the creator of the game who provides Compendium access. The creator can promote any player to GM status, and can even rejoin their game as a player, limited to player powers.