What must I do and what is the ruling on my prayer if I wake up and find my clothes wet with a liquid whose source I do not remember, knowing that the liquid reached the horizontal underpants? And must I change all my clothes, or is it sufficient to change only the inner garments? And what is the ruling if I wish to pray in these clothes? And what is the ruling on madhy (pre-seminal fluid) and the clothes it stained? And can I pray in them after only changing the inner garments?

You did the right thing by performing ghusl for janabah upon waking and finding wetness that you predominantly thought to be semen. Semen is pure according to the more preponderant opinion, and your prayer with underwear that had semen on it is valid. Changing clothes defiled by semen is more preferable and cautious. Madhiy (pre-seminal fluid) is impure, and prayer in clothes affected by it is not valid until they are purified. It is sufficient to change the inner clothes if the madhiy has not extended beyond them.