Saitama's strength in the manga comes from his intense training and unwavering determination. He pushed himself to the limit every day, which eventually led to his overwhelming power.
Boruto's strength in the manga is considerable. He's developed new jutsus and strategies, and has overcome many tough challenges. His potential keeps increasing as the story progresses.
Well, kangaroos have strong legs because they use them a lot for hopping. They start as babies and keep practicing, and over time, their leg muscles grow really big and powerful. Also, their diet helps give them the energy they need for all that jumping.