How he spoke, sat, slept, dressed, walked; How he behaved as a husband, father, nephew; Attitudes to women, children, animals; Business transactions; Stances towards the poor, the oppressed, subordinates, defeated enemies, prisoners of war; Behavior in battle and in preparation for battle; Likes and dislikes; Private dealings with his wives. These are all recorded in documentation.
"Sirat Rasul Allah", a biography by Ibn Ashiq, was written less than three hundred years after his death.
Muhammad is considered by Muslims to be the best moral example of how to live - unlike other historical founders of religions he lived a relatively well-rounded life, with business and political dealings, raising children, fighting battles, compromising principles to the degree required to protect others, etc.. Unlike some such figures he also never claimed to be more than a man. Finally, he lived to old age, and died very normally. His example is thus much easier to emulate than that of, say, Jesus.
Attempts to formulate a new fiqh ("jurisprudence") for the modern era often focus on the sira, asking "what Muhammad would have done" given the modern problems and opportunities.
This is obviously highly speculative, prone to all forms of bias, and totally political. But it is possible to the degree that his intent, actions, and justifications can be understood. He is an historical figure who lived a well-documented life, like Mohandas Gandhi or Augustus or Marcus Aurelius. Thus, the exercise in imitation is possible if not simple.