If you work for a small startup (where you'll be forced to be "senior engineer" if you like it or not), you'll have no "manager", "peers", "projects", "hiring", "mentoring", "networking", and so 95% of the list items immediately pop.
peers = peers
projects = you’d have at least one (and smaller additions to the product that could be projects in their own) hiring = you could be involved in the hiring process even though probably the CTO will handle it
mentoring = you can mentor others or get nentored by e.g the CTO
... etc
so where is that 95%?
Further to this point, networking is crucial too, but it's more networking outside the company. With other startups, with other technical folks -- your hiring pipeline is built on your personal network until you get to at least 20+.