Big qualifier here, I'm not a proper expert on medieval monks/friars. I've done a small amount of reading but not as much as I'd like.

We know quite a bit about Francis of Assisi in a large part thanks to the fact that he was quite famous during his lifetime and the church generally kept the best records in the Middle Ages. Francis himself wasn't literate so we don't have any writings directly from him although we do have a narrated last will and testament. [Edit to add: there appear to be some collections of writings of St. Francis you can buy. I was pretty sure he was illiterate but don't have a proper reference for it right now. These could be works he dictated or I could be wrong.] As is pretty common with medieval figures who rise to fame during their lifetime we don't know much about Francis' early life. We have some accounts but there's not much supporting documentation beyond 'what Francis told X friend of his.' That's to be expected and as far as early lives go we know more about Francis' than we do about a lot of other similar figures.

St. Francis was hugely popular during his lifetime. The Franciscan order started largely as people just following him around and grew so quickly and to such a large size to result in pretty early papal attention. St. Francis did run the risk of being labeled a heretic early on because his order technically broke with conventional church structure at the time. Friars were wandering monks who preached on street corners. Preaching was the job of priests and monks generally weren't meant to wander (not that it didn't happen, it just wasn't common especially by the 12th century). Luckily for Francis he held the Pope in very high esteem and the Pope generally liked that also his huge popularity wasn't something to be denied.

In the end the Pope (I sadly can't remember which one..) assigned Cardinal Ugolino (future Pope Gregory IX) to help provide some proper structure to the Franciscans. Francis had originally run his order on a largely 'do what I do and what I say' system but they had grown so large that was in no way a feasible structure. Ugolino imposed a rule on the Franciscans which involved compromising some of Francis' original beliefs. Francis had believed that the Friars shouldn't have anywhere permanent to live as that would be indulgent but Ugolino set up the system of Chapter Houses which were like monasteries that were technically owned by the Church but run by the Friars (sort of a loop hole so he could say the Friars still didn't own homes). Francis was eventually pushed out of his order for the most part and by the time of his death he was largely uninvolved in the running of the Franciscans.

Still, Francis was a major figure and as an interesting side note he has the record for shortest time between his actual life and his canonization as a saint. St. Francis was canonized by Pope Gregory IX within two years of his death.

Reference: R.W. Southern, Western Society and the Church in the Middle Ages

Edit to add a question of my own: If any Papal historians drop by, and if it's okay to add my own extra question, does anyone have any thoughts on why it took so long for there to be a Pope Francis? I was honestly a little shocked that this guy was Francis I, I'd have thought a major saint like that would have been taken as a Pope name earlier. Were there just no Franciscan Popes or is there a tradition of not taking Medieval Saints as Papal names?