Planning to Switch to Product based company

I work in one of Big 4 companies as a front-end developer. I am currently learning the backend as my project doesn't have any FE work. There are lots of politics now in my team as compared to the previous year, i was denied a well-deserved promotion for some weird reasons, and the hike was low too. Decided to do bare minimum work now and focus on studying for switch.

Could anyone please help on how to get started on preparation for DSA and what other things to focus on to secure a product based company?

YOE -6 Skills - PHP, React JS, JAVA. Have working experience on Automation testing as well.