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There is some disagreement regarding the status of a shimmering portal for a permanent teleportation circle.
My understanding is that the permanent circle has a permanently open shimmering portal.
The contesting opinion is that circle still requires a wizard to use a spell slot to open a portal in the circle.
To me their interpretation defeats the purpose of a permanent circle.
If a random squirrel discovered a permanent teleportation circle, would they see an open portal that they could enter?
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 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.
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....
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.
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.
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.
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.
The purpose of a permanent teleportation circle is to function as an anchor and exit for the circle created during the typical casting of Teleportation Circle. This is why you automatically learn of two permanent circles when you learn the spell; otherwise the spell would be useless.
As the spell is one direction and requires a casting, there is little reason to use a permanent teleportation circle as an entrance for the spell (other than possibly saving on the cost of the chalks). The spell also does not describe the exit circle shimmering or even opening a portal.
The DMG description of Teleportation Circle include the text:
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...
Essentially, there is no mechanism for deciding which circle you would be teleported to without the casting of this spell (or a similar one such as plane shift).
