I will discuss constructions of supersymmetric interfaces in quantum field theories with eight supercharges, both within one theory and between different theories, that provide physical realizations of certain geometric constructions in the context of quiver varieties (such as stable envelopes and actions of Yangians/quantum loop/quantum elliptic algebras). The basic building blocks are supersymmetric Janus interfaces that realize stable envelopes introduced by Maulik and Okounkov. 3d theories give rise to their elliptic versions as in Aganagic-Okounkov, and the K-theoretic and cohomological versions follow by the dimensional reduction to 2d and 1d. Based on the work in progress with N.Nekrasov.