Manual crafting
Many items could be created manually by hand. Manual crafting is mainly required in the beginning of a game and is somewhat usefull to create small amounts of items later on.
Manual crafting has the advantage that all needed ingrediants are created automatically as long as the required resources are in the inventory. So to create an Assembling machine 1 manually it will be queued till the required Iron gear wheels and Electronic circuits are crafted automatically, then the Assembling machine 1 gets crafted.
The disadvantage of manual crafting is that all needed resources needs to be in the inventory and that it could take a long time to mass produce items. Therefore it is adviced to automate as much as possible by using Assembling machines for example.
Some items couldn't be manually crafted like the Engine unit or anything that is produced in Chemical plants.
Some items could currently only be build manually like the Oil refinery or the Rocket defense cause they use more ingredients than Assembling machines could handle. This may be changed in further game versions.