Biphenyl symmetry group dependence on torsional angle
The $20$ degree structure exhibits the $D_2$ point group and the $90$ degree $D_{2d}$.

The figure below shows an $D_{2d}$ structure (a spiropentadiene) but the similarity in symmetry should be clear (I hope ). (From molecule-viewer.com)



The biphenyl at an angle not $90$ degrees is shown below