'Which solver do Googles OR-Tools Modules for CSP and VRP use?

I am currently evaluating googles or-tools and just noticed that it's not really a solver on its own but mainly an interface to other solvers. What I'd like to know is which solvers this framework uses for constraint and routing problems.

I have already looked thoroughly through https://developers.google.com/optimization/, but only found that

for linear optimization Google's "in-house, open-source GLOP" is used
for network flow optimization an own solver seems to be used ("OR-Tools provides several solvers for network flow problems in its graph libraries.")
for mixed integer programming the open-source programm "COIN OR branch&cut" is used by default (but SCIP, GLPK and Gurobi can be integrated)

But on the CP & VRP info/guide sites there is no indication as to what solver is used for these problems...

Does anyone happen to know which solver is used for CSP / VRP or have you found something I overread?

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'Which solver do Googles OR-Tools Modules for CSP and VRP use?

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