We study general settings of the Dirichlet problem, the Neumann problem, and other boundary value problems for equations and
systems of the form $\mathcal{L}^+ A\mathcal{L}u=f$ with general (matrix, generally speaking)
differential operation $\mathcal{L}$ and some linear or non-linear operator $A$ acting in $L^k_2(\Omega)$-spaces.
For these boundary value problems, results on well-posedness, existence and uniqueness of a weak solution are obtained.
As an operator $A$, we consider Nemytskii and integral operators.
The case of operators involving lower-order derivatives is also studied.