Beginner here!Why is it Treasury yields and equities are inversely related, like if stocks go up bonds go down? why this happens?

It's not necessarily so, though there is a time lag.

Interest rates and bond prices however are negatively correlated:

https://www.investopedia.com/articles/fundament...

Here's a simple way to look at it without all the economic/finance jargon.

On the basis that we must be rewarded for the risk we take, equities give more returns than treasuries (which are usually risk-free).

Favourable economic condition -- Outlook for stock performance improves -- Demand for stocks increase -- Price of stock increase -- Demand for bonds decrease -- Price of bond decrease