The JuMP ecosystem
The JuMP project consists of the main JuMP package, and a large collection of supporting Julia packages.
The JuMP source code can be found at https://github.com/jump-dev/JuMP.jl.
Beneath JuMP, there is an abstraction layer called MathOptInterface (source code). MathOptInterface defines an API that solvers should implement so that they can be used by JuMP, handles the automatic reformulation of problems via bridges, and has a large infrastructure for automatically testing solvers.
At the bottom of the stack, there are solver wrappers. These packages wrap the solvers, which are often written in C or C++, and implement the API defined by MathOptInterface.
A non-exhaustive list of solvers available through JuMP is available in the JuMP documentation.