Learn how to use the Erase And Fill tool to delete a part of connected T-Spline geometry and fill new gaps with faces in Fusion.

In the Design workspace, Form contextual environment, select Modify > Erase And Fill
.
Select the faces to be erased and filled.
In the Erase And Fill dialog, set the Fill Hole Mode.
Select OK.
Alt+H / Ctrl+H shortcut for Feature Selection.