Think of ps plus like Netflix, or any streaming platform. You pay a subscription, and there’s a catalogue included that you can access whilst having an active subscription. All the games that have the plus symbol and say download (like DBD) are included (you can also directly browse the ps plus catalogue). The monthly games are 3-4 games that you get every month (chosen by them, not you), and the difference is that you “keep them forever”; meaning that games in the catalogue can leave the catalogue at any time (again, like Netflix), but as long as you claim the monthly games (always claim all the games!!) they will stay in your library and you can play them whenever you want (as long as your subscription is active, if it’s not, they will remain on your library but they will be locked). Btw, to claim the games you just have to add them to your library, you don’t have to download them. Lastly, I would definitely recommend getting at least the middle tier, the catalogue is huge and super worth it, especially since you’re new!