So there are. a few different progressions. One, and the main one, is your collection level. This is the cards you have unlocked. Everyone gets a few of the same basic cards to start. This progresses when you upgrade cards using the credits and boosters. Upgrading cards gives you between 1-8 collection levels, depending on the upgrade rank. Moving up CL will give regular rewards of boosters, credits, gold, and most importantly new cards. Outside the collector token shop (which is irrelevant for you until you u hit 500 CL) this is the only way to get new cards
Boosters and credits are your upgrade card currencies. Boosters are specific to a card, credits are general. You get boosters from each match, win or lose. Credits are mostly unlocked through missions.
Ok the other advancement is the ‘seasons. We are at the very end of the Wakanda season featuring Black Panther. Each day you get 3 sets of 2 missions, and you can have 6 at a time (so think of them as ‘dailies’). These help advance the season. There are also season missions which work the same way but don’t give credits directly. Season advancement is used for variants, boosters, and gold. Now you can buy season passes for $10, but its not required. There are some specific cards that are locked to the season for a period of time, and seasons last a month. The season card (Black Panther this month, Miles Morales last month) does go into the card unlock pool at season end though, so don’t worry if you didn’t buy. Its not gone for good.
As for cards, there are 3 pools. You start in pool 1, which is simpler cards. in the 225 range you move into pool 2, and start unlocking more advanced cards. At like 486 you go to pool 3.
You will unlock all cards off a given pool before you go to the next, though order is random. So don’t worry if you haven’t gotten Spectrum or Nova yet, you will in good time. And its not like pool 1&2 cards are bad. My current deck which is fairly successful even against high level players with full pool 3 decks, is almost entirely pool 1&2 cards, with really just 2 pool 3 cards. Its just pool 3 cards have a lot more advanced strategies and synergies.
TLDR: Complete dailies, upgrade cards, don’t worry about spend. Use gold to unlock more missions early. If you buy anything the starter pack for $3 is good value, and well worth it as a ‘purchase’. Its the only money I spent, less for needing it (its a variant of a pool 1 card anyhow, not something you won’t unlock early) and more because the devs deserved the money.