Clash royale is a very popular pvp strategy game, that i have been playing for many years.
In a normal Clash royale battle you will face a random opponent that has around the same amount of trophies as you. When you win a battle you will gain about 30 trophies, and the higher trophies you have the harder the opponents you face. That is where the upgrading system comes in, winning a battle gives you a chest that you can open for gold and cards. once you get enough cards you can upgrade your troop to the next level for some gold.
One reason Clash royale is so popular is because it has very frequent events and updates. recently they added evolved cards wich are just better versions of the normal cards, they are hard to get but that gives you something to work towards. My favorite event mode is draft where you get to pick from random cards. I like it because its random and people can't p2w.
My only complaint is that Clash royale is the BIGGEST p2w game on earth. Because you can just buy evolved cards, and stuff to upgrade your cards.
Overall in my opinion Clash royale is one of the best card games that I have played. it has a very large variety of cards so you can have a unique deck, face opponents with unique decks, and play styles