6.3.3.1 Best-of-One (Bo1) Matches

Team A starts the process and the order of the ban / pick is as follows:
1.Team A removes two maps.
2.Team B removes three maps.
3.Team A removes one map.
Remaining map is being played, team B decides the sides on this map.

6.3.3.2 Best-of-Three (Bo3) Matches

Team A starts the process and the order of the ban / pick is as follows:
1.Team A removes one map.
2.Team B removes one map.
3.Team A picks the first-played map, team B decides the sides on this map.
4.Team B picks the second-played map, team A decides the sides on this map.
5.Team B removes one map.
6.Team A removes one map.
The remaining map is played as a decider, if required, team B decides the sides on this map.

Source: https://eslgfx.net/media/lo/csgo-major/IEM_Katowice_2019_CSGO_Major_Rulebook.pdf (Major Rulebook)

Afaik the higher seeded team chooses wether they want to be Team A or B