How do u work on In demand Tech?

I have seen companies asking React, Angular, Vue, Next,etc. JS Frameworks for Frontend and Java Spring, Express, Golang, Python on the backend. What about people who work on Legacy Tech Stack used in their teams in bigger MnCs like Amazon, Oracle, Microsoft, Google and others. I came to know they have a lot of Internal Tools which are not used outside like their own Version Control, Java Frameworks and Frontend Tools which won't be used outside. How do u prepare urself with the JD the applying company has asked then? Tc- 32lpa Yoe-1.5