One of the downsides of being able to change lens is the ever present danger of dust getting into the camera and coating the sensitive sensor. Every time the lens is taken off there’s a chance that dust will swirl into the cavity of the camera, attracted by the charged sensor on the back wall behind the mirror.
I took this photo sometime last year but never noticed the dust until later. I had a blower and soft brush with me so it was easy to clean the sensor.
Luckily removing the dust spots is fairly easy in Lightroom or any photo development application but this was easily the most dust I ever saw and was a rain pain to go over the whole image. (made easier by page up/down and the “visualise spots” setting, but still..)