Connect water filter to water supply
Basically, any unit can be connected to the water supply in many ways, but each system is designed or intended for a specific connection method. Direct Flow systems require a 3/8" hose, which can handle large flow rate.
Even if this video is in danish, I hope you learn something.
The easiest and most comfortable solution: Connect to the faucet with this adapter. Video about the adapter
The larger systems are supplied with this adapter and can be connected to a water pipe and a corresponding ball valve that can be either to a 1/4" or a 3/8" hose.
With the small valve with the handle you open/close for the water to the osmosis system.
On my sink faucet in the bathroom, I have this connectivity option for hand shower. This connection is seen in many of the videos.
At the mixer thread with an adapter