This looks like it might be twig blight, a fungal disease.   Here is some information from the Pacific Northwest Plant Disease Handbook - https://pnwhandbooks.org/plantdisease/host-disease/juniper-juniperus-spp-twig-blight.   This site refers to juniper but is the same fungal disease that can affect Arborvitae.   It lists several cultural controls you can try like pruning out & burning affected twigs & branches.

Here is another source of information from Washington State Extension that should be helpful - https://hortsense.cahnrs.wsu.edu/fact-sheet/arborvitae-twig-blight/.    It also discusses non-chemical (cultural controls) and chemical management options, e.g., fungicide application.