6) How are delegates to conventions selected?

Answer 1

Delegates are elected.
the election process is determined by State and local law.

Answer 2

Depends on the party, but the Dems and Republicans choose delegates to the National conventions at their District and State conventions. Delegates to the State Conventions are chosen by the local County parties and District parties.
Third Parties have various ways of doing it. For them, on the local and state levels, it is whomever really shows up to the meeting usually. On the national level, while they may apportion delegate counts, often they allow whoever of their members that show up to participate.