IBIS automatically change shutter speed

This is a generic question, but I am more of a landscape photographer than anything else, and I do not see a tech support forum.

I have been using a Canon 6D with a Tamron 28-300 for many years and am looking at jumping into mirrorless. This lens has image stabilization that I can really see working through the viewfinder before I take a picture, but the camera always chooses a shutter speed that is at or one increment slower than the reciprocal of the focal length. I have never paid much attention to this or tried to determine how much slower I could go if I picked my own shutter speed. Maybe the body does not know that the lens has image stabilization and I have been missing out all this time.

I know that IBIS in the most recent bodies can reduce the shutter speed required, but my question is whether it does on its own. For example, some lens + body combos are supposed to provide up to 8 stops of stabilization. I have seen reviews test this and claim that they could only get high success rates with (let's say) 4 stops of stabilization. If I shoot aperture priority mode, how much slower than the reciprocal should I expect the shutter speed to be?

Varies based on what it determines it can get away with (intelligent)?

Never any slower and I need to shoot manual mode (dumb)?

Depends on the manufacturer?

Do some manufacturers allow you to bias the shutter speed without manually setting it?