I am planning to purchase the master tier subscription. I have read over it and checked the faq but I am still unclear on how it works. Do my players have access to all of my digital content? I plan to purchase it so my players can look up monsters and read all my source books even the DMG if they so choose in case they wanted to try their hand at DMing a oneshot or something. Is this possible? Will they also be able to access the spells, items and monsters sections of the site to quickly look up and filter things? 

Yes, to all your questions.

Also, the Master Tier works the other way around as well. You can access content not owned by you but purchased by one of your players.

Do I grant players access by creating a campaign and sending them the invite link or is there a special way to grant this access?

You create a campaign, send the link, then you just need to toggle the box on the top left of the campaign page.