My layman's understanding, based on just having finished reading Brian Greene's "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory" [1], which was recently a Prime Reading selection (free to borrow for Prime Members), is that is that yes, it started out as one theory, with 10 dimensions, and then other forms were discovered, until they had 5 different string theories. That was the situation around 1985.

It's important to note that the equations of string theory (both when it was one theory and when it was 5 theories were beyond our ability to solve exactly. We could only approximate them, originally with things called "perturbation methods".

Perturbation methods involve solving something for the big influences, then adding in corrections to deal with smaller influences. For example, suppose you wanted to calculate the orbit of a small moon in a close orbit around a large planet. You could first calculate what the orbit would be in a universe that just has that planet and moon--we can do that exactly. Then you could add in the small differences that would be caused by their sun and other planets in their system--the perturbations caused by the sun and other planets.

If, however, you had a planet with two large moons, you might not be able to solve it for just the planet and one moon, then make small corrections for the other moon and the sun, because the influences of the other moon is not small. The exact solution for one planet and moon is just too far off from the final solution for the former to be tweakable into the later.

There's a thing called the "string coupling constant". Perturbation methods can only be used in the 5 string theories when the coupling constant is less than 1. Eventually they found out how to solve some problems in each of the theories even with coupling constants larger than 1, and to the surprise of most they found that the 5 theories could be split into 3 groups.

One group contained two of the 5 theories. A universe following one of those theories with a coupling constant of, say 0.5, would be identical to a universe following the other theory with a coupling constant of 2.0. The two theories are said to be duals of each other.

Another group contained just one theory. That theory was dual of itself. That is, a universe following that theory with a coupling constant of 0.5 would be identical to a universe following that theory with a constant of 2.0.

The final group contained the remaining two of the 5 theories. I'm having trouble remembering how those were connected.

Anyway, somewhere along figuring out that last group, it was found that low energy point-particle approximations of some of the string theories gave some of the 10-dimenensional supergravity theories that had been worked on before string theory to try to unify gravity and QM.

Then in 1995 Witten showed that if you took some of the string theories and go from low to high coupling constant, the physic you get has a low energy approximation that matches 11-dimensional supergravity.

Anyway, where they got to from there was that the 5 string theories had come out 10-dimensional due to the fact that they could only approximately solve the equations, and that string theory actually was an 11-dimensional theory, and the 5 string 10-dimensional theories (and 11-dimenensional supergravity) are just different perspectives on the one 11-dimensional theory that you get when you make certain approximations or take it to certain limits.

(I may have botched an arbitrarily large amount of the above, so caution is called for)

[1] https://www.amazon.com/Elegant-Universe-Superstrings-Dimensi...