Collabs happen when you mix two max level weapons.

Most of the collab rankings was measured in terms of how much it was used for leaderboard runs back in 0.6. While there were numerous changes from 0.6 -> 0.7, the meta for DPS seems to have stayed regardless of which characters you pick depending on the archetype they build into (tank, runner, special). Tank uses jing builds while Runners go for glass cannon Holy Fire builds and Idol Live users just abuse the broken special of certain characters