If it's below the meter, from the utilities side. I'm thinking you will likely need to discuss it with the local power companies engineer. As they don't go by the NEC.
Anything on the utility's side of the service meter will require the utility's input on how you do it. The utilities aren't governed by the NEC, so they will either have their own standards that apply in addition to the NEC, or standards that are less conservative than the NEC.
Between the meters and the corresponding service disconnect(s), generally speaking, this is in the customer's scope of ownership, so the NEC will apply directly. As of NEC2020, a new rule now requires any splices or tap connectors that are on the supply-side of the service disconnect to be listed for it.
Once you are load-side of the service disconnect, any listed connector for the specifics of the application, can be used for splicing and tapping conductors. The standard version of these connectors need to be inside an enclosure that protects them from rain if installed outdoors. The submersible versions of these connectors are rated for being used inside handholes, where they could be submerged in water.