Starborn: Nesta vs. Az (ACOSF & HOFAS spoilers)
I thought HOFAS made it obvious that Nesta is Starborn, while simultaneously shutting down any possible theories that Az might be, but I've seen people claiming the opposite. So, let's explore the evidence.
Starborn Tattoo: Nesta had the eight-pointed star tattoo on her back. Az did not. We know the eight-pointed star tattoo was chosen by the magic when Nesta made her bargain with Cassian, and we know it's the symbol of the Starborn because Nesta told Bryce that it looked just like her scar, which is also the same eight-pointed star in the Prison chamber. Az could have easily been the one with this tattoo in ACOSF - he could have made a bargain with Cassian where the magic chose that design - but yet he wasn't. Nesta is the only ACOTAR character who had the Starborn logo tattooed on her.
Starborn Chamber: Nesta found the chamber in the Prison, whereas Az, in his 500 years, never knew that it existed. Bryce mentioned in HOFAS that someone could have only found the chamber if their starlight guided them to it. This proves that Nesta has starlight - the magic of the Starborn - as Nesta is the only ACOTAR character to find the Starborn chamber.
Canonically, Nesta and Bryce are the only characters to be able to enter the chamber. Az could only enter when Nesta opened up the wards to allow him in. This, in and of itself, proves that Az isn't Starborn because if he was then he could have entered without Nesta's assistance. Nesta is the only ACOTAR character to be able to enter the Starborn chamber.
Dread Trove: Nesta found, can summon, wield, and control the Harp and the rest of the Dread Trove, whereas Az cannot. I mention the Harp specifically because Bryce said that the Harp was left for a Starborn to find. The last person that used the Harp, prior to Nesta, was Silene and the last person to wield the Horn, prior to Bryce, was Helena, which supports the theory that Nesta is a descendant of Queen Theia and Fionn. Even though ACOSF said that Made people can wield the Trove, that doesn't seem to be the case since Theia, Helena, and Silene wielded them and they're not Made. Plus, CC said that the Starborn, with their starlight channeled into the Trove, can bring the objects to life. Nesta is the only ACOTAR character who found, can summon, wield, and control the Dread Trove. Queen Briallyn wielded the Crown (to an extent) because of Koschei, but the Crown seemed to choose Nesta over Briallyn since it did nothing to protect the queen from being UnMade.
Gwydion and Truth-Teller: We know both blades answer to the Starborn bloodline by emanating with magic for them and them alone. Neither blade emanated with magic for Az, proving yet again that he is not Starborn. Simultaneously, SJM made sure that Nesta touched neither blade at the same time. Nesta wielded Ataraxia in the tunnels, Bryce laid Truth-Teller on the table, and only handed Nesta Gwydion right before she shut the portal. The lengths SJM went to to make sure we wouldn't have confirmation of Nesta being Starborn just further proves that Nesta is. SJM could have easily shut down any potential theories by having Nesta touch both blades and them not emanating with magic.
Both blades also left Az's hands when Bryce beckoned for them because, once again, she's Starborn and he is not, and the blades always answer the call of a Starborn.
Az is definitely something - a descendent of Enalius perhaps? - but he's not Starborn. All signs point towards Nesta being Starborn, either the descendent of Queen Theia on Silene's side or made Starborn via the Mother while she was in the Cauldron.