Population III stars are believed to be some of the first stars to form in the universe. So, what makes them so special they need to be categorized separately from Population II stars? Possibly nothing. There are two ideas as to what exactly Population III stars did. The first has been proven to be true: they formed from gas gatherings when the universe was first generating. These stars would have created the first metals; however, they would not be as special as to need a separate classification from Population II stars.
The second idea is that they created the dark matter in the universe, specifically baryonic dark matter. This would warrant a separate classification because Population I and II stars do not do this. While it has not be proven that Population III stars did this, it is a possible theory as to what influences the creation of dark matter.
So how did these stars form in the first place? When the universe first formed, many dense gas clouds were created. One theory states that the gas and dust would gravitate together, creating a denser cloud that would slowly form chunks of matter and build up into increasingly larger objects until stars formed. The pancake theory states that objects began in clusters, which then separated and regrouped due to gravitational pulls. Moth theories have a number of issues. For starters, the groups formed would have been significantly smaller than the galaxies we know today. Another issue is figuring out what happened to the particles left behind after these events occurred.
What did they look like? No one knows for sure. There are ideas that they could have been smaller stars resulting from how hydrogen molecules were forming in the early universe. Larger star theories suggest they could have taken a bigger shape due to the Cosmic Microwave Background or a lack of metal. Some suggest there could have been a mixture and smaller stars many have orbited the larger ones. And where did they go? As far as we know they may have collapsed and became black holes, or they may remain and Lyman-alpha clouds (Image 1).
These stars could have formed before or after the first galaxies took shape. If they did before, then they spread out in the universe. If they formed after, they likely became trapped in those galaxies. They are made of gas formed during the Big Bang, specifically hydrogen (Image 2), helium (Image 4), lithium (Image 3), and beryllium (Image 5). They were the first stars and as a result, had almost no metal within their composition; however, they did produce metal.
No Population III star has ever been found as far as we know. Part of this could be that most of the bright ones have already died. We would have a much harder time spotting the ones that are less luminous and as a result it would be far more difficult to locate a Population III star. There also could be many that have attracted gas and metal to them over time, causing them to appear similar to a Population II star with low metal content. We mentioned briefly that they produce metal in the core. If that metal rose to the surface, it would also cause the star to blend in as a Population II star. If every single Population III star had a high mass, it is most likely they have all burned out.
References
Carr, B. J. (n.d.). Cosmology, Population III. NASA/IPAC Extragalactic Database. Retrieved Apr 19, 2022 from https://ned.ipac.caltech.edu/
Population III. (n.d.). Cosmos. Retrieved April 19, 2022 from https://astronomy.swin.edu.au/cosmos/P/Population+III
Mysteries of Space 
