Dust visible in the viewfinder is usually on the underside of the focusing screen, above the mirror. Not actually inside the viewfinder, though that can happen, and not on the mirror or on/in the lens - dust in those places is too far out of focus to see. Use a rocket blower (£10 Amazon) but be careful do not touch anything inside the mirror box. The focusing screen surface itself is very delicate and easy to mark.
Dust on the sensor is not visible through the viewfinder, only in live view at a high f/number (lens stopped down) or in photos. Eg darker spots on a plain sky taken at say f/16. A well-targeted rocket blower will usually shift those too. Bear in mind that the image on the sensor is inverted, so dust in say the top right corner of the image will be on the bottom right of the sensor as you look at it.
Unfortunately a bit of dust is a fact of life, but rarely much of a problem.