Hi Michelle,
1. Once a contact engages any of marketing activities, he will be counted as Marketing Contact.
E.g: John Doe is created from a landing page, then he will be counted as a marketing contact, then 1 contact is used by the organization.
However, if we send marketing email to him later(Or he submits other form), total marketing contact will still be 1.
2. Scoring Model only applies to leads. By default, when a score reaches the sales-ready threshold, a plug-in automatically sets the sales-ready flag on the lead to True.

Then a workflow called "Lead to Opportunity Stage Transition" will be triggered accordingly, which automatically moves the lead to the sales-acceptance stage(final stage) of the lead lifecycle.
You can customize this workflow as needed such as qualifying the lead to an opportunity.
Please refer to this section:
https://docs.microsoft.com/en-us/dynamics365/marketing/score-manage-leads#establish-grades-and-the-sales-ready-score
There are other configurable settings for Scoring Model:
https://docs.microsoft.com/en-us/dynamics365/marketing/mkt-settings-lead-score-options
In a word, it will only move lead to sales-acceptance stage and mark the flag field, qualification process can be added by us manually.
3. By default there isn't process on creation of lead, the lead will be created by System if he is created from a marketing page/form, and be assigned to Administrator.
We can assign lead to others if our security roles have sufficient Assign privileges on Lead entity.
It can be said that Sales app and Marketing App are just different interface, but they share same processes or workflows.
4. Only lists can be used in CRM/customer journey.(In other word, list entity only.)
Therefore, you should store list members of the third party vendor as contact records, then create a segment/subscription list to find these contacts to engage them with customer journey.
(You can import external data via Data Import Wizard
https://arcticit.com/2020/06/getting-the-most-out-of-the-dynamics-365-data-import-wizard/)
5. It will report on Insights tab of marketing email record:
When an address returns a hard bounce, Dynamics 365 Marketing will automatically stop sending new messages to that address for six months, but your email results will still show each attempt as a hard bounce.
https://docs.microsoft.com/en-us/dynamics365/marketing/insights-glossary