I have seen platforms like Trendlyne share portfolios of well-known investors like the ones below.
https://trendlyne.com/portfolio/superstar-shareholders/index/
Radhakishan Damani
Rakesh Jhunjhunwala
Sunil Singhania
Mukul Agrawal
Ashish Kacholia
Mohnish Pabrai
Nemish S Shah
Vijay Kedia
Dolly Khanna
Porinju Veliyath
How do these platforms get access to this shareholding data. I have seen this data on a lot of news portals as well.
If you are a shareholder you can write to RTA (Registrar and transfer agent companies) which maintains the shareholding list of public listed companies asking for the complete sharing holding list. You could also directly write to the company as a shareholder asking for this information. By the way, the shareholding list will be in millions for companies like Reliance. 
All listed companies also declare anyone holding more than 1% shareholding in their quarterly filing with the exchange. This is available on the exchange website itself.
You can also download the entire shareholding for any public or private limited company from the MCA website (updated once a year). This costs around Rs 100 every time you have to download any information.
The public information available on the portfolio of these star investors is taken from the above three sources. As you might have realised, it potentially could be delayed.
These big investors usually buy and sell large quantities in the block deal window on the exchanges. NSE and BSE publish a list of all such deals. You can keep track of the buying & selling activity on the exchange websites.
https://www1.nseindia.com/products/content/equities/equities/bulk.htm
https://www.bseindia.com/markets/equity/eqreports/bulk_deals.aspx