How does the Democratic Party's policy-making procedure work?
The Labour Party in the UK has quite a byzantine policy-making procedure and committee structure that tries to ensure all the stakeholders have a say in drawing up policy, but (to oversimplify massively) the best way for most people to influence Labour Party policy is by joining their local Labour Party and/or affiliated Trade Union if appropriate, attending meetings to vote on resolutions, and by getting themselves elected to various local, regional, and national committees.
I'll speak as a member of my local county Democratic Party. County parties elect delegates to the state (Georgia) party convention. In turn, the state convention elects delegates to the national party convention. At both levels, delegates participate in writing the party platform. The platform is a manifesto of what the party wants to accomplish going forward. Individual candidates are not bound by the platform, they may differ in some respects. But if they differ too much, they may not get political and financial support from the party.
Both Georgia and nationally we hold primary elections. The Georgia primary will select the party candidates for partisan offices - delegates have no say. If nobody gets over 50% of the votes, a runoff will be held among the top two. Whoever wins the primary then goes against the republican candidate on Nov 3rd. Other states may differ in how they select candidates.
The Georgia delegates to the national convention, along with each of the other states and districts, are assigned to presidential candidates proportionally to primary votes with two provisos: a candidate has to actually qualify to be on the primary ballot, and candidates who get less than 15% of the votes are considered "non-viable" and get no delegates. If you accumulate more than half of the delegates before the convention, as Biden nearly has, you win the nomination on the first voting round. If not, the "superdelegates" get to participate in later rounds. These include current congressional and statewide elected officials, plus state and national party officials, and a few past office-holders (president, Vice-president, congressional leaders). Superdelegates account for 15% of the total.
Again, Biden, assuming he becomes the nominee, is not bound by the party platform. However, by virtue of having the majority of delegates participating in writing the platform, it will be very close to his personal policies.