Wa alaykum assalam,
We can all, as Muslims leave a valuable legacy for our Ummah.
Of course, ensuring we remain firm upon the commands of Allah swt and follow the example of the Prophet :saws: will enable us to succeed in every other aspect of our life.
Setting up organisations or charities is one way, yes, but it is important to keep in mind that there are countless ways we can serve the Ummah - basically by focusing on our own abilities and aiming for perfection in every way we can, doing the best in everything we do. It's not always about focusing directly on 'changing the world' or other large-scale operations, but first changing ourselves, setting our own affairs aright, and then the affairs of those around us and our community. If we all did this at the grassroots level, then it would inevitably result in large-scale change. Getting the foundation - that community involving the family, teachers, neighbours etc - established will make the whole structure firm. And yes of course when inshaAllah one ever get married and has children, they should raise them to be pious individuals who will only benefit the ummah further, this is extremely important - this applies to brothers as well
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btw, here's another video I quite liked, by Nouman Ali Khan, it is around 40 minutes long but so worth it inshaAllah:
