The Defenders, back in the day, didn’t include Luke Cage and Danny Rand. Dr Strange originally formed the team to defend threats on a grander scale, while Power Man and Iron Fist formed the Heroes for Hire and fought street-level villainy. That all changed in the mid 90’s when Dr Druid took over the Secret Defenders and shuffled the roster. Oddly enough, it also included Drax the Destroyer and USAgent. That lineup was short lived, but Netflix shuffled the deck with their assorted Marvel shows, and the lineup was codified in comic continuity in 2017.
Luke was subjected to assorted unethical experimental treatments in prison, and walked out with unbreakable skin and enhanced strength. A fairly pedestrian power set, the strength (hah!) of the character has always been who he is, rather than what he can do. More grounded that most, Luke’s outlook on life and commitment to doing what’s right rather than what’s expedient (or even what’s legal) has made him one of the more engaging characters in comics today, and even landed him the leadership role in the New Avengers after Captain America was assassinated (he got better…)
Danny’s past is a little more complicated and honestly, doesn’t sit as well with modern audiences as it did in the less-culturally-sensitive 70’s. As great as some of Iron Fist’s arcs have been (especially the Immortal Iron Fist arc from the mid 2000’s that introduced the Seven Cities of Heaven and their champions), the “white saviour” trope is a thing. Danny’s an orphaned millionaire who was raised in the mysterious city of K’un- Lun and succeeded in defeating the dragon Shou-Lao the Undying to become the Iron Fist, a master of martial arts who can channel his chi into his fists to strike with explosive force. It wasn’t until the aforementioned Immortal Iron Fist series that we got to get a real look at -why- K’un-Lun had a ritual challenge that threw their temple students against a dragon, and insight into the champions that came before Danny. I highly recommend the series.