What makes a self-made website safe from getting stolen (Hacked)?

Hello experts, honestly, I'm thinking of creating a website, an important website, from zero. I admit it will take me years to be fully built professional so I'm trying to know some basics and learn well before starting a single step. I was wondering, like YouTube got thousands of employees working in it,... show more Hello experts, honestly, I'm thinking of creating a website, an important website, from zero. I admit it will take me years to be fully built professional so I'm trying to know some basics and learn well before starting a single step.

I was wondering, like YouTube got thousands of employees working in it, what makes YouTube safe from unexpected bad intentions from theses thousands, like what are these workers meant to do exactly?

What is the substantial thing in YouTube, Facebook, Google, Twitch websites (for example) that if bad hands reached it, the website will be (lost, hacked, stolen)?

Warmest regards for all experts who are devoted in helping who need.
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