When a donor submits a gift, they will receive the specific Confirmation you have set to deliver per giving Page or Form settings (or, if you don't have one specified in Page or Form settings, your default Donation Confirmation). Once the donor exists in your system, they will have access to the Donor Portal.
To learn more about how a donor can interact with the Donor Portal, check out this Support Article.
A best practice you can lean into is including a link directly to the Donor Portal in your Confirmation Emails, as demonstrated below.
Organizations Using RaiseDonors and Virtuous CRM
If your organization is using RaiseDonors and VCRM in tandem, you will most likely be processing all your receipts out of VCRM. To learn more about pushing Annual Giving statements processed in VCRM to the RaiseDonors Donor Portal, check out this support article.
To learn more about managing the Donor Portal when using RaiseDonors and Virtuous CRM, check out the details in this support article.