There are 2 theories that are used to understand molecular geometries:
Valence bond theory (VBT)
VBT considers that atoms approach each other to form a molecule. The bond formed results from the overlap of atomic orbitals resulting in a bonding orbital with the electrons localized between two atoms.
This theory is also used to explain hybridization
Molecular orbital theory (MOT)
MOT assumes that when atomic orbitals overlap, the overlap results in the formation of a new orbital called molecular orbital
A hybrid orbital results from the mixing of different types of atomic orbitals on the same atom