Avoinna ja suljettu?
I've just learnt the words auki and kiinni (again, I learnt them a long time ago) but I previously came across the words avoinna and suljettu, and the phrase menevät kiinni.
Are they interchangeable? or is there a difference in meaning/grammar?
To add to Zzzzz...'s point: In principle, suljettu means the place has been closed (by someone) and won't reopen, whereas kiinni doesn't say anything about the future (and also not about someone having done something -- in another context it could be something having become stuck to something, for example).
In practice, don't expect even native speakers to follow this rule...
is suljettu the past tense - just as you were saying it's something completed already. It's been closed. Whereas avoinna (not sure what form the ending "na " means) is in the present. Is it saying it's "now open".
I don't suppose you'd get these 2 words together on a sign.
You're on to something: suljettu is not only an adjective but apparently also the past passive participle of the verb sulkea (to close).
Avoinna, on the other hand, is an adverb that originates from avoimena, the essive (answering As what?) of avoin (open).
Here you have a nice poster using both: https://taitokeskipohjanmaa.fi/taitokeskus-sievi-kesan-aukioloajat-avoinna-3-6-20-6-suljettu-21-6-14/.