The simple answer is directly to the silica surface by silicon to carbon bonds
For more details, the Cogent UDA™ HPLC column ligand is a C11 chain with a carboxylic acid on one end. On the other end, there are two orientations for how the ligand could be bonded to the Cogent TYPE-C™ Silica support.
The first orientation is shown in A. In this case, only one hydrosilation occurs from the starting reagent, resulting in a remaining double bond and a single point of attachment.
In the second orientation B, another hydrosilation occurs with structure A and a nearby Si-H group, resulting in no more double bonds. This requires that the other Si-H group is close enough to react. This is called a Bidentate attachment.
For the Cogent UDA™ HPLC columns, B is structure of the ligand.