Q.Russian arborvitae
What are good companion plants for Russian arborvitae on a slope?

Since Russian Cypress are a lower growing plant, you can look at plants both taller or ground covers for your planting area. Consider the soil and sunlight and your growing zone.
Visit local garden centers or greenhouses for plants suited to your region.
https://www.gardeningknowhow.com/ornamental/shrubs/russian-cypress/russian-cypress-care.htm
https://www.gardeningknowhow.com/special/slope-hill/plants-that-grow-on-slopes.htm
https://www.gardeningknowhow.com/special/slope-hill/hill-ground-cover.htm