The "Phantom Impact" code is the best way to get primo gems for free, besides playing the game and farming up commissions, chests, and quests. miHoYo will be sending out a new live stream with each version upgrade, with characters and events details. It's an incentive to watch the livestream, but you should be able to redeem them at any time anyway.

Version 3.8 will feature a number of characters that are rumored to be reruns, including Eula, Clay, Cocomi, and Wanderer. We will add each "Phantom Revelations 3.8" live streaming code as they are dropped during the program period. These codes typically expire within one day.

You need to be Adventure Rank 10 to redeem the code, otherwise there are two ways: first is the code in-game:

The other way is from the official website: