In ranked season, you are matched up with opponents that are close to your placement in the division. The longer you let the bar go, the more likely you are to get matched up with someone much higher or lower than you are. You have control over this and can keep the range low if you want. As you win consecutive games your division will increase, and the competition will get better..
In BR, it is based on how far you are within the run. You could match up with the best player in the world if it is his first game in that run.
Overall XP level and the types of cards you are using mean nothing. Rating mod based on the cards you are using used to factor in, but that was removed years ago.
You don't need to worry about spending actual money on packs or cards. For the past few years it has been really easy to build a quality team for free between moments, conquest, free packs, inning reward programs, and other pathways. Once the game gets rolling you can ask for more specific advice about which cards to target first.