The best way to learn how to start your own company is to join an early stage where they give you lots of responsibility and ownership. The closer you are to the founders the easier it is to learn from their mistakes.
This isn’t really true. The important experiences at a startup are meeting and learning to work with investors, the board, regulators, early key customers and so on. As an employee you won’t get this exposure (tho’ the founders will dangle it in front of you like a carrot). As an early employee your job is to grind, sacrifice your health and relationships, then get diluted out by the next fundraising round.