Can we use sender authentication private domain?

 I need to verify a domain to use different senders in Marketing Cloud (companydomain.com). We already have a Sender Authentication Package that is a subdomain of our domain (sap.company domain.com), but we need to use some emails as sender which they are from our domain (user1@companydomain.com, user2@companydomain.com …). These senders change a lot and sometimes we need to use Dynamic Senders, so I saw two options: use either a Private Domain or a Domain Registration. Can anybody help me with two doubts about it:

1. Private Domains allows you to send mail from an authenticated domain (https://help.salesforce.com/articleView?id=000318538&language=en_US&type=1&mode=1) This means that the send with a sender from a registered a Domain will not be authenticated or it depends on the configuration in our domain?

2. Can We use our domain as a private domain? Or it needs to be a subdomain (a non-existing domain)?