The Special Purpose LLC (SP LLC) is an asset of your Solo 401k plan. The sole member of the LLC is your 401k trust, and you are the manager. The SP LLC has its own tax iD number, and its own bank account.
Funds go first into your Solo 401k bank or brokerage account via rollovers or contributions. You then invest 401k funds into the LLC (an asset of your 401k trust).
Once funds are moved from the 401k bank account to the LLC bank account, the LLC can hold its own investments. This provides some separation between the 401k assets and those in your SP LLC.
Important: You cannot use a pre-existing LLC to hold Solo 401k-owned assets. The Special Purpose LLC must be a newly formed entity.