https://forums.frontier.co.uk/showthread.php?t=246308
Who has beta access:
Beta access is usually closed access and limited to those who have the lifetime expansion pass (either premium backer or separate purchase) or those who have purchased the season of content with beta access. An example of this is Season 2 Horizons which was sold with and without beta access.
For more details on who has what access see
here.
Once the beta is live it can be synced and updated via the launcher and shows up separately on the launcher. This occurs identically on the steam launcher where applicable. Note, some large beta updates require manual downloading of the new launcher from Frontiers store, this is usually announced in advance.