Promotion cards are included in box purchases or can be purchased individually by specific sellers. They were introduced with the release of the 5m01 box. Due to their success they have been included in subsequent 2 yuan , 5 yuan, and 10 yuan sets. They come in individual silver packets. Each pack contains one promo card and there is no way to tell which set the inside promotional card is from.
Promotional cards are tied to specific sets, not all sets have promotional cards.
These cards were intended by the manufacturer to be distributed with each eligible box purchase but most sellers do not include the promotional packs. GoddessTCG.com has an agreement with certain sellers on Aliexpress to include promotional packs with eligible boxes. More details can be found here.