Each building has a number of upgrades you can do to improve its efficiency. 
The Level of your building, which you can see in the building overview, represents how many upgrades you have completed. This makes it easier to see which building can still be improved, and which ones you have already improved to the max.

 

What things you can upgrade depends a bit on the type of building, but some possible upgrades are:
  1. ⚙️ Unlock additional production lines up to three
  2. ⏱️ Increased production speed of production lines.
  3. 📦 Increased  product storage of production lines.
  4. 🔧 Increased retooling speed of production lines.
  5. 🚚💨 Increased shipping area
  6. 🅿️ Unlock additional parking space
  7. 📋 Unlock additional resource options

 

TIP: If a building is upgraded to the maximum, it will display max instead of a level number in the building overview. To get an overview of what still needs to be upgraded, you'll find the upgrade overview in the same place as the building overview.