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You can use various official streaming platforms where you can watch movies legally. Some popular platforms are:

Netflix (Hollywood, Bollywood, Tamil, Telugu, Bengali)
Amazon Prime Video (Hollywood, Bollywood, South Indian movies)
Disney+ Hotstar (Bollywood, Hollywood, South Indian movies)
ZEE5 (Bollywood, Bengali, South Indian movies)
SonyLIV (Bollywood and South Indian movies)
Hoichoi (Bangla movies and web series)
MX Player (Free Bengali and other Indian movies)
YouTube Movies (There are many free and paid movies)
2. Government and free platforms
There are some free websites and apps where you can watch movies legally:

YouTube (Free Movies Section)
Voot (Indian Movies & Shows)
JioCinema (Free movies and web series, for Jio users)
Plex (Free Hollywood movies)
3. Television and online TV channels
Free movies can be watched on the official apps of some TV channels, such as:

Star Gold, Sony Max, Zee Cinema, Jalsha Movies, etc.
Hotstar (has some free movies)
ZEE5 (free section)
4. Libraries and official websites
Many government and educational organizations have movie archives, where some old movies are available for free:

Internet Archive (archive.org)
Public Domain Torrents (legal torrent movies)