How to create the Api endpoint to connect Jira with other systems?

Welcome to the Atlassian Community!

You don't create any end-point in Jira.  They already exist automatically.

To "get" the endpoint you want to use, you will need to read the documentation over at https://developer.atlassian.com/cloud/jira/platform/rest/v3/intro/ to work out what the end-point you want to hit is, and then issue a network call to it.  It will then return what you asked for (or an error if it can't service the call)

For example:

curl --request GET \ --url 'https://your-domain.atlassian.net/rest/api/3/issue/ABC-123' \ --user 'email@example.com:<api_token>' \ --header 'Accept: application/json'

Will give you a JSON representation of all the data on the issue ABC-123

Note the bit about "api token" - you will need to read through https://developer.atlassian.com/cloud/jira/platform/rest/v3/intro/#authentication to work out the api token to use.