From what I've been able to gather, the very bright spot at the center of galaxies is produced by both the central quasar and a dense cluster of millions of stars that surround it. How much of the central light is produced by the quasar, and how much comes from the star cluster?
Only a few percent of galaxies in the local universe have active galactic nuclei (e.g., Mishra & Dai 2020). The fraction with optically visible AGN (e.g., Seyfert nuclei) is lower than this, since in some galaxies only the X-ray or radio emission gets through the dust in the nuclear region of the galaxy. Even when the AGN is directly visible in optical light, it is usually significantly fainter than the rest of the galaxy. True quasars are quite rare.
So in most cases, the "very bright spot at the center" is produced by a dense accumulation of stars, either in a bulge of some kind or in a compact nuclear star cluster. This is the case for M101 (e.g., Kormendy et al. 2010), the galaxy whose image you presented in your question.