The Digimon Card Game is a competitive physical TCG with real depth, and players who know it will find Alysion's rules familiar. For everyone else, the Story Mode's approach is the intended onboarding path - learn by playing against an opponent who is teaching you as you go.
Both players alternate turns drawing and playing cards, and the goal is to reduce the opponent's Life to zero. The twist is that there is no summoning sickness in the conventional sense and no resource cost to playing a Digimon - the constraint that shapes everything is memory, which acts as both your turn action and your hand limit.
Memory is the defining system of the Digimon Card Game. Players place memory cards from a shared memory gauge, and it functions as a turn action, a hand limit, and the thing that lets you evolve a Digimon from one form to another. Spending memory to bring up a stronger Digimon is the core tempo decision of every game, and the reason a player with more cards on board can still lose.
In the digital version this carries over unchanged, which is good news for anyone migrating from physical play and a genuine depth test for anyone arriving fresh.
Digimon evolve along defined lines, and reaching a stronger form costs memory rather than being automatic. Evolution is where most of the game's decisions concentrate - a Digimon on board is worth less than the same Digimon at its final stage, but the path to that stage costs you tempo and memory you might need elsewhere. The digivolution animations in Alysion are a presentation flourish over a system that was already the most interesting part of the physical game.
Decks are built around an archetype - a named strategy with its own card support and its own game plan. This is where deck building stops being about card count and starts being about plan. A deck that wants to fill the memory gauge fast and one that wants to control the board play very differently against each other, and knowing which archetype you are piloting tells you what your opening turns should look like.
Story Mode is the sensible starting point, because it introduces the systems in the order the game expects them and gives you real opponents to test against. The Digimon Liberator webcomic and light novel are both free to read on the official site and provide the narrative context if you want it - though nothing in the card game requires reading them to play well.