I think this is probably dependent on the club and coach.
Speaking only for my son's club and coach, they make no guarantees for play time at a specific position, but the coach will work with the players to try to get them play time at the position they want. However, in some cases that might mean on a lower tier team. I know one player, for example, who I understand (second hand) was invited to continue on the team in his main position, or continue with the club in the position he wanted to move to, but on the lower tier team in the second case (as he was not as skilled/valuable at that position). He ended up staying and playing at his initial/primary position on the higher tier team, and now splits time between both, as he is developing.
Re starting lineup, this varies a bit from game to game, and depends on other factors somewhat (eg: attendance at training, performance in the last week, injuries, etc.). I wouldn't assume an offer to join a team/club comes with an expectation of being in the starting lineup; that's generally something you would earn on an ongoing basis. For example, my son has started most games for his team, but there have been a few where he started on the bench, even as a "starter" level player for the team.