What's In a Golden Pack
A Golden Pack contains six cards, all of which are rare or mythic rare from a Standard-legal set and have duplicate protection. At least one card will be mythic rare, and each other card has an independent chance of being mythic rare approximately 1 in 8 times. At least two cards will be from the latest Standard-legal set (currently, The Brothers' War).
Golden Packs will not have cards replaced with wildcards and do not affect the odds of packs opened later containing wildcards. However, opening Golden Packs does advance your wildcard tracks.
If you would receive a card but you have four copies of all cards of that rarity from that Standard-legal set, you will instead receive a card from another Standard-legal set. Only when you have four copies of all cards of that rarity from each set in Standard will cards be replaced by gems.
How to Get Golden Packs
Each The Brothers' War pack purchased from the MTG Arena Store progresses your Golden Pack meter one step. The meter does not advance when packs are acquired through means other than pack purchases. When all ten steps of the meter are filled, it resets and a Golden Pack is added to your collection, which you can open on the Packs scene.
If you purchase a pack bundle that contains more packs than steps left in your Golden Pack meter, it will continue to progress after it has reset with the remaining number of packs in the bundle. For instance, if you have five steps already filled on your meter and you buy a bundle containing ten The Brothers' War packs, your Golden Pack meter will fill, reset, and then fill five more steps.