To build a lot of the things in this mod is kinda ridiculous but it was not really a huge issue until we needed to make barrels. Can someone explain to me why a barrel is made out of a small tank and a one way valve?
Like to get a barrel you need to two medium 1 tanks, then smack them together to make a medium tank 2 then cut them up to two small tanks and put a valve on it to get a barrel...that makes no sense.

A barrel isn't made out of a small tank. Many barrels are made out of a small tank. An individual barrel works out to be quite cheap in terms of resources and crafting time, regardless of the steps involved. And once you add productivity modules, the more steps the cheaper it becomes. The number of outputs on Nullius recipes are very often something other than 1, so if you don't pay attention to that, then none of the costs will make sense. Fuel canisters also give multiple per craft, so each medium tank is basically a full stack of 50 canisters.

Factorio recipes are an abstraction. In Nullius they represent shared manufacturing processes rather than literally turning a completely finished product into another finished product. When two things are largely made in similar ways from similar materials, the commonality between them is reflected by using one as an intermediate for the other.

These aren't just ordinary, run of the mill barrels. They transport acids, bases, steam, compressed gases, and all kinds of highly reactive chemicals. They're very high quality barrels. If you want to talk about things making sense, you should probably avoid transporting sulfuric acid in vanilla barrels made of 100% iron.

You need tanks as an intermediate for all sorts of things. They should be automated as a priority. If you don't have them automated by the time you need barrels for experimental chemistry, it's as good of a time to get started as any. Prior to that, if you're just in need of some for canisters, you can hand craft a bunch pretty quickly, especially if you pop in some crafting equipment.

Factory Planner/Helmod are nice tools, but they can encourage people to think in terms of building an entire production chain all the way from raw materials to finished product, which is rarely an ideal approach in Nullius. Barrels do not require a lot of steps. They require 1 or 2 steps to make from the tanks that you need anyway for lots of other stuff. The production for tanks should be shared, not duplicated a dozen times across your factory for everything that needs tanks.