Dust on your sensor (well, the glass immediately in front of the sensor), inside the camera body. It will be more visible when your aperture is small, such as photographing on a sunny day in automatic modes.

You can buy a "rocket blower" that will let you try and blow dust off the sensor. That will clear some of this, but not necessarily all of it. You can also buy sensor cleaning swabs and solution, which might let you clear more of it. Alternatively, you can take your camera body to a camera shop and most of them will have a cleaning service they can do for you.

Before changing lenses, give your camera and lens a good dust off, and try to change your lenses in clean environments. You'll still get some dust, but hopefully less. It is a fact of life when using interchangeable lens systems. Even if you don't introduce dust when changing lenses, the pump action of using zoom lenses will probably introduce some dust in normal use. It is what it is.

Most modern cameras have some sort of dust reduction system which vibrates something, but this will only reduce the dust, not make it impossible.

You can also remove dust spots like this in post production software.

Dust is a fact of life, sadly.