I wanted to have a detail about auto-suppression (HasOptedOutOfEmail in core/salesforce object value) prevents journey emails from being sent out in marketing cloud. So, doest having the value of HasOptedOutOfEmail true makes email sendinng from journey impossible? Thank you
No it doesn't. For example, you can create a Transactional Send Classification that ignores the subscription status of your contacts.
So, if you have an email in your Journey that is tied to that Send Classification, even if your contact has HasOptedOutOfEmail = true, he will receive your email.
Auto-suppression list is another story. You don't even need to have the contact in your system. You just have to add email addresses that you don't want to receive emails from your org.
Suppression lists filter out email addresses and prevent those addresses from receiving Email Studio messages. Suppression lists serve as a do-not-contact list for your email sends. Auto-suppression lists typically include addresses with a history of spam complaints, unsubscribe lists from previous providers or advertisers, addresses of your competitors, and canceled customers.