Preferred vs common only matters when the company is sold at loss? The preferred shareholders get priority in terms of being made whole before the common shareholders.
In this case, since series C was 1.5bln and sold for $975m, then the preferred shares were bought during C would be made whole first (assuming the prior rounds were made whole first and there's enough left over for series C preferred) before those who exercised right after the valuation for common shares.
Edit: I forgot that preferred shares also come with liquidation preferences too, meaning that in a loss situation, later employees are highly unlikely to get something
In this case, since series C was 1.5bln and sold for $975m, then the preferred shares were bought during C would be made whole first (assuming the prior rounds were made whole first and there's enough left over for series C preferred) before those who exercised right after the valuation for common shares.
Edit: I forgot that preferred shares also come with liquidation preferences too, meaning that in a loss situation, later employees are highly unlikely to get something