This is the 658th most frequent Latvian word.
bijušas
The Latvian word "bijušas" means "have been" (feminine plural, past participle of "būt").
In this sentence, 'bijušas' relates to a past participle describing the state of the feminine plural noun 'buildings' as having been built.
Daudzas skaistas ēkas ir bijušas celtas šajā pilsētā.
Many beautiful buildings have been built in this city.
Here, 'bijušas' reflects the state of 'things', which is a feminine plural noun, as having been true.
Šķiet, ka tās lietas ir bijušas patiesas.
It seems that those things have been true.
The word 'bijušas' indicates a past perfect state attributed to a group of females.