Can I access and use content shared by other players in the campaign?

Yes. If you are in a campaign where any player has a Master Tier subscription with content sharing enabled, the shared content of all players will appear in Maps as it does in other parts of D&D Beyond, and you can access that shared content. You do not need to own it yourself. However, only Master Tier subscriber DMs can upload their own custom maps and access their own homebrew monsters.