I would try the very simplest cleaning routine: using a nice, clean bulb blower (like the Giottos Rocket
https://www.bhphotovideo.com/c/product/259157-REG/Giottos_AA1900_Rocket_Air_Blower.html), you take the lens off, hold the camera with the lens mount facing down, and gently give a few puffs onto the sensor from an inch or so away hoping to dislodge and make the dirt drop away from the sensor. That will usually clean up anything that isn't sticky/stuck to the sensor cover glass.
For anything like that, you need to use a cleaning solution and sterile/clean applicator, which is when you decide whether it's worth $50 to let someone already practiced at it do the job and take the risks, or you learn to do it yourself and take your own risks.
😉
I've been in the habit of dusting out my camera bodies with a Rocket almost every other time I pick them up, and as a result I've never had anything get stuck to any of my cameras' sensors.
G