I'm gonna break it down so we don't get lost.

• The Warp is effectively a sea of emotional energy
• Due to millions of years worth of negativity, this energy is largely tainted
• Warp exposure mutates flesh, inorganic matter and minds. Flesh & materials start to alter and twist, minds start to become more and more tainted by negative and excessive emotions
• Demons are 'living' concentrations of warp power
• When a Demon possesses or is bound into someone or something, it starts to affect the object/person both mentally and physically
• When a demon is purposefully bound into someone by someone else, they're referred to as a Daemonhost. Daemonhosts have sigils preventing them from becoming changed too much so the person holding the Daemon doesn't break apart under the onslaught of Warp energy

TL;DR: Warp mutation can happen both through exposure to the Warp, as well as through possession. In the former case it's random what you get, in the latter case your mutations will reflect the nature of the demon possessing you.

Generally speaking Warp mutation happens quite a bit slower. Like being X-Ray'd a lot vs swallowing a ball of Uranium. In the literature some objects/people mutated over the course of decades if the dodged outright possession and exposure was relatively low.

I hope this makes sense.