How do you research/unlock conveyor tubes?

Trying to connect a basic refinery and basic assembler but conveyors are locked. It says I need to research them. The in game help and info for anything really is abysmal.

Help please.

 

You have "progression" enabled in your advanced settings for the game. Build a small cargo container (I always build a small-grid version that uses much fewer parts). Once you've done that, progression will allow you to build conveyor tubes.

 

A

A small one? Ok cheers, I'll try that. I've built a medium one but not a small one.
--- Automerge ---
Sorted. Made a small one. Turned out I never finished making the medium container coz I lacked cobalt for the metal grid.

On a side note, how do you figure out what unlocks what please. The progression tree is clear as mud.