Where this rendered reports are stored?
Is rendered reports are stored in MongoDB?
In the blob storage. Typically the same place as your templates, but it depends on your configuration. See what you have in the blobStorage prop.
Why the rendered reports can be access from the url at any time?How long the rendered reports can be access?
The cleanup is optional. See the docs for the config options
https://jsreport.net/learn/reports#cleanup
If it is stored in mongodb then is db size of mongodb will increase right?
Yes. There is cleanup config to make it maintainable.
Is rendered reports templates are removed , while delete templates of report from studio?
No.