Have you ever played Go Fish or Solitaire or poker? The concept of a "deck" is essentially the same, except with those older games, the deck is always the same 52 cards (+/- jokers and rules cards, depending how much attention anyone is paying :P). It's a pool of cards you use in particular match. In card games like Gwent, Legends, etc. you instead "build" a deck out of the cards you own, which you get from the big pool of all cards in the game via random packs. Once you've built a deck, you can take that deck and play matches against other people / AI with it - once you're in the match, the deck is locked in, you draw from it, play from it, discard from it, etc., all cards you have access to in that match are pulled from the deck you built.
Most games let you build multiple decks just to have, so you can easily chose to play a different one next match if you want. Nothing that happens in any of the matches in any of these games can ever affect what cards you own, and will not affect your deck after the match is done. I.e. even if you go through your entire deck in 1 match, you can then play the next match using the same deck and it would start with whatever you built it as in the first place.
The cards presented to you at the start of a match is just your starting hand. That "redraw" is called a lot of things, the one I'm most familiar with is "mulligan", all that is doing is letting you have some control over your initial draw so that affects what you can do your first turn. Cards you chose to redraw will get shuffled back into your deck. Other mechanics vary a lot per game, as others have said, and I have been largely disappointed with digital card games so I can't speak too much to those :P
