What are some tips for someone who wants to make career switch from a US technology company to indian space and defense research organizations like ISRO, DRDO etc.?

If you want to do good research and be paid well at the same time, this is not a suggested career move.

I was a former employee of BARC and my advice is- don't come back for joining DRDO or BARC- they are not research institution, rather bureaucratic government office where there is nothing enthusiastic for juniors in terms of work culture or job satisfaction. Here seniority is given priority over innovation/excellent research achievement and there will be a lot of seniors to eat up your work credit and if they fail to do so,  they will make your life hell by suppressing all your promotions and career advancement opportunities. Competence means nothing if you can't oil your seniors for getting all of your rightful things starting from even getting a sitting chair. Work culture here is worst, people stop studying after they get job here and nobody is bothered or competent enough for good research work or taking relevant projects and finishing them in time. Don't know about ISRO as I didn't work there but reading their achievements it seems that ISRO has good work culture compared to BARC or DRDO.