Maps selection system

Can someone explain please how does the map pick/ban system works? Specifically the decider map for the BO3

in a Bo3 its usually (in main events I think)
A ban map
B ban map
A pick map
B pick map
2 maps remain the more popular is played, I think teams vote for maps before maps start, then the average of all teams choice is like sorted. the more popular between the 2 maps will be played.
in open qualf higher seed gets to pick i think
also there is gonna be a new map soon after that we get a new veto, probably same as csgo

In BO3s,
Team 1 bans 1 map
Team 2 bans 1 map
Team 2 picks map 1
Team 1 picks side for map 1
Team 1 picks map 2
Team 2 picks side for map 2

The final map is the highest-seeded map. Teams send their preferences and the map is decided by the highest choice and Team 2 chooses the side for Map 3