Is it okay to say :
This building is built by company A.
I thought this is okay because the sentense delivers truth. But I am not sure since I haven't seen many examples of built with present tense.
This building was built by company A.
But the action of building happened in the past, so past tense seems correct.
Which tense is correct? Or are both correct?
The second example is straightforward, it's the passivisation of "company A built this building". Very straightforward. No question of what it means.
The first example is more complex. It can be parsed in multiple ways. It could be the passivisation of the present tense:
Company A builds this building.
That's not actually a structure you'll come across that often. If the building is going on now, it would be "is building", and if the building were finished it would be "built". If there were several buildings, or a category of building, and there's a long-running scheme of building them all, you could have:
Company A builds these buildings.
But that that's not what we're dealing with. That sort of present tense in the singular, with a specific building, will mostly be seen on a timeline, referring to either the past or the future, where people are listing events with when they happened or are expected to happen.
However, that's not the only way to parse it. I can find little formal support for this, with dictionaries not having such things as examples that I've yet been able to find, but I know from experience that it is reasonably common usage to use "built by X" as a participle phrase, making it function as an adjectival phrase. There, built is the past participle, and is built up into a participle phrase with the prepositional phrase "by X", and the whole phrase becomes an adjective.
I don't know what any formal grammar would make of that, but it is a usage I've come across - and even used - several times in my life, and that's the best way I can make sense of it. Taken that way, "built by company A" is effectively an adjective, in your first example, and the use of is is completely natural.
