There are two ways to connect somebody to an opportunity, and they are opposites.
Assigning approves them directly, skipping your recruitment workflow entirely. Sharing the link sends them through that workflow like any other applicant. Which one you want depends on whether you already know enough about the person.

Only Vome Users are notified when they are assigned. Offline profiles are added to the opportunity but receive nothing, because there is no account to notify, so telling them is on you.
From your Database, which is the faster route when you are handling more than a couple of people.
You can assign more than one opportunity in a single pass, which is useful when somebody is joining several programmes at once.
Yes. Open the profile from your Database and use the Opportunities section on it to assign opportunities to that person. It is the same action, reached from the other direction.
Share that link however you like. When somebody opens it and presses to apply, the opportunity's recruitment workflow decides what happens next, which might be a form, a request for your approval, automatic approval, or nothing at all if they are not eligible.
Yes, and it is usually the better answer for people you already know. Instead of assigning them by hand every time, add a rule to the opportunity's recruitment workflow that recognises them and approves them automatically.
A rule with a condition such as approved for the same category, or finished a sequence, ending in Auto-approve, means your existing people register themselves and are in immediately, while strangers still get the full form. You stop doing the sorting.