I wanted get my passwords stored in Passwords Manager how to achive that uisng CURL request.
If I understood correctly and you are using Passwords App, then there is an API description, read it and use with CURL:
I also recommend checking out the Postman collection which allows you to inspect api calls: Working with the API · Wiki · nextcloud / passwords · GitLab
Yes they have api documetation but it has only part of url metioned eg: /api/1.0/password/list
But how to athenticate with api using curl and what is the exact curl command.
Check this tread Create a list of photos with a specific "Tag" - #5 by gas85 authentication is the same and usually you have to set header to use CURL.
curl --insecure -X GET -u username:password 'https://xxx.xxx.com/api/1.0/password/list
Not getting any output