Grit is a branch book, exploring one core idea, namely that of "Grit" over and over again from different perspectives.
The main takeaway is that the character quality of grit - others might call it persistence, perseverance or resilience - in Finland they call it Sisu, is the main predictor of success. If you can face problems and bad times with equanimity and push through the pain, that's the one important ingredient necessary to set up for success in life.
Because over time the people who show up and work through the bad times, no matter the pushback they get, will eventually overcome any kind of obstacle. This is essentially what Elon Musk does. Some ideas of the book are linked to ideas from Mindset by Carol Dweck because Grit is a character quality that people with a "Growth Mindset" have more of than people with a "Fixed Mindset". Hard things, especially those that you can not solve immediately are a source of constant and painful frustration for people without grit. They might even give up on solving them, coming up with excuses why the problem is not worth solving, after all, to put themselves in a better light. For people with grit, they are an obstacle to overcome no matter what. A challenge to be taken. Everything worth attaining, be that a career as an artist, in academia or sports, or maybe even founding a startup contains myriads of challenges and usually high chances of repeated failure, that's why people with high grit excel at them. In the end, the message is - not talent is what matters, but constancy and persistence for which you need Grit in the face of adversity. Often the way to get Grit is to have a strong passion for something. When you love something and do it only for the sake of it, you don't mind the pain, the setbacks and the struggle, because the passion for the thing is stronger than all of that. Shallow external motivation, on the other hand, is usually not enough to persist.
A lot of what Angela Duckworth writes about in the end is much older and nothing new. Old stoic ideas like those you would find in Marcus Aurelius's Meditations or in places like the Tao Te Ching) repackaged under a new name, with more modern examples, targeted more towards entrepreneurs, founders and startup people.