Arcade 100k medals are awarded in the Arcade when you get a high enough score in any game (unless Auto-play was used). You receive one 100k medal per 100,000 score earned in a game, multiplied by the cost of the game in arcade tokens. (This means that a score of 1,000,000 on Triangle Count will give 30 100k Medals, for example). This means games that tend to have higher scores (ie. Triangle Count) will provide more 100k medals than games that have lower scores such as Ultimate Avoidance.
100k medals let you buy upgrades that help you in the arcade games (No effect in Ranked mode) or you can spend them on the general items: 25% boost and max boost for all, 1.5x Arcade XP/Coins (1 game), and unlimited arcade tokens (1 day). 100k medals are also used to feed the pet .