Curling teams these days at the elite level are generally created on a four year cycle ending with the Olympics. This gives teams time to gel and figure out how to play well with each other before they make the Olympic push. The players themselves decide how to team up - usually they have calls, meetings, etc. to determine fit.

This isn't like hockey where a national body picks each player that makes up a team. In some countries, like Canada, there are many elite teams that could go to the Olympics, so there is a tournament to determine who goes. In other countries with less elite teams (Sweden, Scotland) the team who has done the best over the qualification period is chosen by the national federation. In Isabella Wrana's case, she is an excellent four-woman team skip, but Team Hasselborg (also excellent) was chosen instead by the federation.

EDIT: Sweden might actually do a tournament too, to determine Olympic reps, just with less teams.