Hello @Alexandru Lungu,
Thank you for reaching out to Atlassian Community!
When using Jira and Confluence under the same domain, users created will belong to that domain as well, so yes, if you invite a user you can allow them to access Jira and Confluence or only one of the products. All users will be listed under the User management page.
Regarding the storage, each product has its own storage, so attachments added to a Service Management ticket won't affect Jira Software storage and vice-versa. You can find the details on Cog icon > User management > Storage.
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