Since that tutorial was made, the Geometry Nodes system has radically changed. It now uses 'Fields', which are evaluated and interpolated at every requested coordinate of a given space, rather like shader nodes.
Given a vector-space, a texture node is now implicitly sampled at the relevant points , when an evaluation is asked of it:
Here, the Wave Texture node is evaluated at the 'Positions' (Equivalent to 'Object' Texture Coordinates in shader nodes) of the subdivided input Geometry, automatically interpolated between its vertices. (The Position input node is actually optional, here, it is the default.)
If you prefer learning from videos, I would suggest, at least to start with, you seek out the ones made after 'Fields' were introduced.