"that elementary particles formed by strings?"
Elementary particles are described theoretically by the standard model a well developed and flexible quantum field theory, with the group structure of SU(3)xSU(2)xU(1).
The interest in string theories arises because this group structure can be embedded in the group behavior of the vibrations of a string. Automatically all the success of the standard model can be reproduced by string theories in their mathematical construction, assuming that the elementary particles are vibrations on a string,( of any type as long as they can carry the group structure).
In a sense , if strings had been studied before the experimental observation of SU(3)xSU(2)xU(1) , they could have predicted it.
It is not in the equations , but in the solutions, elementary particles are identified with the vibrations of the string of any type.
This post imported from StackExchange Physics at 2020-12-03 13:04 (UTC), posted by SE-user anna v