This is a custom design for a basic homemade air gun, using a garden hose handle as a firing valve.
This is a slightly more advanced homemade design, though it still works off very basic principles.
The only reason it's complicated is the insertion of the 1/2" PVC End caps, which require sanding and balanced, brute force; the use of 1/4"OD Vinyl tubing and the modification of end caps this requires; the irregular bonding of the breech and tank via an external adhesion; and the attachment of the barrel connectors, which you'll have to customize to your particular hose handle model.
But given you understand how this is applied, it's still incredibly simple. It works off the same "Fill, open valve, fire" operation as any basic homemade air gun.
Remember, this design is here solely to inspire, not to set a design in stone.
One of the advantages of homemade air guns are that they are just that: Homemade And thus very customizable. So have fun with this design.