You could probably find it somewhere on youtube.
In short, in Arena you were presented with unrestricted RNG card draws from all factions. During the selection process you could pick 1 card out of 3 offered to you, and then sequentially build the deck this way, card after card. So, we were building the strategy as the draws were unfolding. Due to RNG it would not be possible to create a uniform cultists deck or reavers deck. Instead, we had to think and consider synergies with every new drawn card to adjust our choices as the deck was populated.
RNG from different factions also resulted in very interesting combos. It was fun to play because every deck created by players was unique and not to be ever repeated again.
After we have assembled the deck, we had to play 9 rounds against random players with their own unique decks. Progressing after the 3rd game was rewarded and winning all 9 rounds was rewarded by legendary animated card, ~200 meteorite ore and ~200 normal ore, as I remember.
The only drawback to Arena mode was that there was no limit on provisions or legendary cards. So, in some games we could have 20 legendaries and in some just 10. The players called for this balancing, but instead the devs replaced Arena with Draft where all cards are pre-selected, as you know.
Now that the devs are going to abandon the game, Arena mode is what's required for Gwent to survive for longer, thanks to unique combinations and fresh experiences.
