All industrial materials in Cities Skylines are transported using vehicles, so conveyors and pipes are just for looks don't actually function. But you can use Building Spawn Points mod and hide some roads inside large buildings to replicate the functionality of pipes and conveyors.

Warning: If your trucks can only spawn at a storage building and that building is full, your extractors will complain about not enough buyers for products. So I'd recommend leaving the cargo trucks spawning at the default spawn point unless you're really sure none of the storage buildings will ever get too full (or be destroyed in disasters/fires).

Warning 2: If you want to remove a spawn point connection and go back to using trucks on the road (or you just forgot to add a connection between two buildings) when using More Effective Transfer Manager, be aware that that connection may have been added to the 'exclusion list' and will never attempt to match again. I think you can only solve this by replacing the buildings entirely.

Building Spawn Points by macsergey: https://steamcommunity.com/sharedfile...
Industrial Pipeline by Badi_Dea: https://steamcommunity.com/sharedfile...
Conveyor belt small by Avanya: https://steamcommunity.com/sharedfile...
Pipes Prop Pack by Avanya: https://steamcommunity.com/sharedfile...
More Oil collection by Avanya: https://steamcommunity.com/sharedfile...

0:00 Introduction
0:20 Adding the pipes (cosmetic only)
2:58 Making the pipes "function" with Building Spawn Points
4:42 Traffic after
4:50 Conveyors

I should also mention that there is a workshop creation called Real Oil Pipelines (https://steamcommunity.com/sharedfile...) which uses an invisible truck running along a narrow road which is modelled to look like a pipeline. It's just a bit of a pain to detail with pipes that are roads, and to control whether the visible or invisible trucks spawn (in Advanced Vehicle Options you can control which ones spawn in your city overall, but you might need to use Vehicle Master Control by Kylte45 to actually control things at the district level).