The portal is not always open
A teleportation circle and the shimmering portal sometimes associated with them are not the same thing at all, although they are related.
A permanent teleportation circle is an inscribed circle with unique sigil sequence used as a teleportation destination. The teleportation circle spell teleports you to a permanent teleportation circle, and opens a "shimmering portal". Not all spells that can use a permanent teleportation circle open a portal; for instance, the plane shift and teleport spells can use a permanent teleportation circle but do not produce a portal, shimmering or otherwise.
The shimmering portal
The teleportation circle spell uses a temporary circle drawn while casting that connects to a permanent teleportation circle by means of a shimmering portal, which is only open briefly (emphasis added):
As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle of your choice whose sigil sequence you know and that is on the same plane of existence as you. A shimmering portal opens within the circle you drew and remains open until the end of your next turn.
Permanent teleportation circles
The teleportation circle spell describes a permanent teleportation circle:
Many major temples, guilds, and other important places have permanent teleportation circles inscribed somewhere within their confines. Each such circle includes a unique sigil sequence--a string of magical runes arranged in a particular pattern.
The Dungeon Master's Guide has additional information:
The presence of permanent teleportation circles in major cities helps cement their important place in the economy of a fantasy world. Spells such as plane shift, teleport, and teleportation circle connect with these circles....
Creation of a permanent teleportation circle
A permanent teleportation circle can be created with the teleportation circle spell:
You can create a permanent teleportation circle by casting this spell in the same location every day for one year.
Using a permanent teleportation circle with plane shift or teleport
As referenced in the DMG quote above, the plane shift and teleport spells also use permanent teleportation circles, but neither spell requires one.
The plane shift spell can use a permanent teleportation circle to specify a particular location on the destination plane:
Alternatively, if you know the sigil sequence of a teleportation circle on another plane of existence, this spell can take you to that circle.
The teleport spell teleports you to a "destination"; one such destination is a "permanent circle":
"Permanent circle" means a permanent teleportation circle whose sigil sequence you know.
But neither spell opens a portal
Neither spell describes opening a portal the way the teleportation circle spell does: plane shift just says you "are transported", and teleport says you are instantly transported. Note also both spells have a duration of Instantaneous, as opposed to teleportation circle with a duration of 1 round.
Other uses of a permanent teleportation circle
There could be other spells, magic items, or other magical effects that make use of permanent teleportation circles. Whether transportation involved a portal, shimmering or not, would be determined by that spell, magic item, or magical effect.