Anyone tell me that where the user data (like photos, videos, posts) is saved? Because I don't think that they goes in SQL or any similar database.
Typically you will use a service like AWS S3 for media (photos, videos, files, etc.). Blog posts (just text) will be in the database in a text type. Media is usually renamed to a unique value and then referenced in the database.
However if this is a personal project not meant for public use you could store media locally in the filesystem (NodeJS, PHP)