The organization portal is the page belonging to an organization you are linked to. Think of it as that organization's own noticeboard: everything it is currently recruiting for, in one place, with the parts that are relevant to you already filtered in.
It is where you go to look around rather than to do your assigned work. Your Home page shows what you are already signed up for. The portal shows what else is available.
Two routes, and both land in the same place.
From your Home page. Each organization you are linked to has a card. Click View portal at the bottom of the card.
From your Profile. Scroll to your linked organizations, find the one you want, and click View portal beside its name.

Browse and sign up. The portal lists the opportunities the organization is recruiting for, and you can narrow them down in whichever way suits you:
Open any opportunity that interests you and sign up from its own page. Depending on how the organization set it up, the button reads Apply or Register.
Yes, and that is the main reason the portal exists.
Being linked to an organization does not mean you are approved for everything it runs. The portal shows you what you are eligible to see, which is usually more than what you are already approved for, so you can put yourself forward for something new without waiting to be invited.
What happens after you sign up depends on the opportunity. Some add you straight away, others send your request to a coordinator, and others start you on a short set of onboarding steps first.
Because organizations control who each opportunity is visible to. An opportunity can be limited by site, by age, by the programs you have completed, or by tags on your profile.
If you believe you should be seeing something, contact the organization directly. They control visibility, so Vome cannot open it up for you.
Those two are yours, and they only appear once you are signed in and linked to the organization.
Both are scoped to the organization whose portal you are on. If you are linked to more than one, each portal shows only its own.
Some organizations add a general sign-up form to their page. When they have, a GET INVOLVED banner appears at the top, and it opens that form.
It is for expressing general interest rather than signing up for one specific opportunity. If you are already linked to the organization, you usually want a specific opportunity instead.