The Difference Between Ninshu and Ninjutsu in Naruto
Ninshu and Ninjutsu are often mistaken as the same, despite being fundamentally different. Here's an overview of how one is different from the other.

Naruto is considered one of the most popular anime and manga series in the world. We all have a special memory attached to the series. Released in the year 2002, the anime series continued till 2017 and became the most popular in the world.
The series is versatile, to say the least, with its variety of characters, clans of shinobi, plots twists, backstories, tragedies, and so on. The series introduced us to many legendary shinobi and their Jutsu.
The history behind how the shinobi world came into existence was not known by many until the Fourth Great Shinobi War was about to end. When the truth came out, people learned about a new form of using Chakra using a technique called Ninshu.

The Sage of the Six Paths, Hagoromo Otsutsuki, taught the people his era the ways of the Ninshu. People used it to make their lives better and eventually as time passed, Ninshu ceased to exist. Instead, people have another way of using chakra called Ninjutsu.
Despite the knowledge of Ninshu being shared during the war, it was still never revived in the modern shinobi world. As the concept behind Ninshu and Ninjutsu is the same, it always creates confusion among fans that both of them are more or less the same. However, they are fundamentally different in every other aspect.
Ninshu

Ninshu was basically a creed or a shinobi sect that strive to achieve an era of peace. People followed it like a religion to both understand as well as teach the mystery of Chakra. Hagoromo started teaching Ninshu as a means of helping people understand themselves better.
They used chakra to connect their inner spiritual and physical energies together instead of each other. Hagoromo believed that everyone should have the equal amount of chakra. If one had it more than the other, then they would become arrogant and would desire even more power.

This is why, instead of using Ninhsu to hurt other people, Hagoromo tried to make it a means of uniting them. So, his teachings minimized the devastating potential that people with chakra can achieve.
Instead, he taught people to only use their spiritual energies and not their physical powers. However, despite his teachings, he ultimately had to use Ninshu against his mother Kaguya to save mankind.

During the war when Naruto was conversing the remnants of Hagoromo's Chakra, the former referred to him as the create of Ninjutsu.
It was a common misconception that Hagoromo was the one and only creator of Ninjutsu. The Sage strongly disagreed with Naruto, saying he only ever taught Ninshu to humans.
And that the humans corrupted his teachings and weaponized something that was meant to be pure and sacred. Hagoromo replied to him how his teachings were different from Ninjtsu.
He was shown to have incredible pride for Ninshu and considered Ninjutsu was something used for evil purposes.

Ninjutsu

To put it simply, Ninjutsu is a weaponized form of Ninshu. As one of the three main categories of Jutsu, Ninjutsu has the same purpose as Genjutsu and Taijutsu since all three are used for offensive purposes.
Compared to the other categories of Jutsu, the concept of Ninjutsu is rather vague. It has the most variety of subclassification like medical ninjutsu, barrier ninjutsu, sensory ninjutsu, and so on. Unlike Ninshu, Ninjutsu was used to strengthen people, making one stronger or weaker than the other.
Ninutsu requires both spiritual and physical energies, making their attacks all the more powerful. According to Hagoromo, Ninjutsu is the corrupted use of Ninshu by humans. It was first created by Hagoromo's elder son Indra Otsutsuki.

Another way of describing Ninjutsu is that i is constantly developing and progressing with the change in time. People are coming up with new and creative ideas to weaponize chakra. Genius shinobi like Tobirama develop their own Jutsu and share their knowledge with their allies.
Over time, far from creating an era of peace, humans started using Ninjutsu as a means of war. For centuries, the conflicts between shinobi due to their land, lineage, and other benefits, the war never seemed to end.
