The length of the list CoefficientArrays is one more than the total degree of polys.
The are sparse arrays with ranks .
The first element has the same length as the list polys.
If polys is a single polynomial rather than a list, is also not a list.
For linear equations, the solution to Thread is given by LinearSolve.
For nonlinear equations, the are not unique. CoefficientArrays by default assigns nonzero coefficients only to monomials where the variables appear in the same order as vars.
CoefficientArrays[polys, vars, Symmetric->True] makes all the symmetric in all their indices. The resulting arrays will generally be less sparse.