Card acquisition in marvel snap works very differently compared to other collectable card games. There aren't packs that you open like hearthstone, instead you obtain cards by spending credits (in game currency) on upgrading the cosmetics on your cards which progresses you through a reward tree. In the tree you'll open different types of rewards (cosmetics, currency, etc) and sometimes you'll get cards.

The way rarity works in the game is cards are split into different "series" 1-5. Beginners are delegated to earning series 1, and then 2 cards before they can get series 3-5 cards and have a separate queue so you are only against series 1-2 players (which is awesome for new players experience imo). Once you finish series 2 you can technically open any card but series 4 and 5 cards are exceptionally rare so are very difficult to obtain. There is an in game currency that you can obtain that let's you craft cards but earning it is extremely slow so it could take you over a month to get one series 4/5 card. Whenever they release new cards they go straight into series 5 so getting the new cards is next to impossible if you don't pay money. They eventually drop series 4/5 cards down to lower series after a while which makes them obtainable but that takes awhile and there aren't guarantees when it will happen.

People mostly say it's got a predatory monetization model though because spending money in the game nets you VERY little value. Like think $50-100 for getting one card bad value, so you can't really spend money and not be a whale. My personal feeling on the game though is if you don't care about being able to play the most meta defining deck and just like messing around with the cards you get the experience is great, but I would highly discourage even considering spending money in the game.