So this question has been bugging me for a while, and I'm getting quite frustrated.
How did people back in the ancient time make sure their buildings/ temples/ roads so they were accurate? That means level foundations, right angle walls, etc. Sure, I've heard of all the tools: The Groma, the libella, the chorobates, the plumb-bob, the A-frame level etc. But to create all these tools you'd still need a right angle or a straight line to make them work as intended. So how did they make these tools without having precision tools in the first place? If I try to cut a board of wood to use in the construction of one of the mentioned tools, it's not going to be perfectly straight/ even. (Human error) And that would mean that my tool also won't give me a straight line/ even plane. Now, I've heard of the string method, that if you stretch a cord/ string it creates a perfect line between two points, but how would you make sure these two points are even/ on the same level? Wouldn't you just get a straight, yet uneven line? You can't use that to check if a surface is level, only if it's straight or not.
It's a similar thing with the right angle. Of course, you can create a perfect right angle by making a perpendicular bisector. But that would still only work well if the surface your doing it on is even. There seems to be a missing link, and I can't quite put my finger on it.
If I described things bad or in a confusing way, then please let me know. If you have an idea about a different subreddit, where I might get a definitve answer then please let me know.
Still, thank you very much for taking the time.
I answered parts of this in response to a related question a while back: the question and answer there were more focused on the tools themselves and how to use them, although I did go into a little detail on how to make them.
To elaborate a little more in specific response to your question:
A plumb bob requires no bootstrapping, just a string, a weight, and gravity. Unless disturbed by an outside force, the string of a plumb bob at rest will form a straight vertical line.
Similarly, the surface of undisturbed still water will form a level horizontal plane. If your chorobates is properly constructed, the groove will fill evenly with water at some alignment. If it doesn't, then you get it at even as possible and then work down the high points.
You can use Pythagorean triples, such as the 3-4-5 right triangle, to construct a right angle. Despite the modern association of these with Pythagoras (c. 500 BC), there's evidence that some cultures knew dimensions of specific right triangles quite a bit earlier, e.g. the Babylonian Plimpton 322 tablet, c. 1800 BC.
Even without knowledge of Pythagorean triples, you can construct right angles by trial and error. Build or draw a rectangle by eye, then measure the two diagonals with a ruler, a compass, or a piece of string. If the rectangle is square, the two diagonals will be equal. If it's uneven, you can adjust the orientation of the sides until it's close enough for your purposes.
You can verify that a piece is straight and even on a given axis by sighting along it. For example, you can hold a board end-on near your eye and look down the length. Any deviation from a straight line will be readily apparent from such a vantage. For a surface, you can either move around it and sight from multiple angles, or you can lay a known-straight board on the surface and spin it around making sure it lays flat along the full length.