1. A method, comprising: establishing, in a set of multi-cloud elastic extension (MCEE) engines, an MCEE logical structure relating at least one tenant resource of at least one tenant site, at least one resource of a first non-tenant cloud at a first non-tenant cloud site, and at least one resource of a second non-tenant cloud at a second non-tenant cloud site; mapping, by the set of MCEE engines, nodes of the MCEE logical structure to a segmented end-to-end virtual network structure (E2E-VNS) such that a collection of resources at each node of the MCEE logical structure is in a separate virtual network of the E2E-VNS; establishing, by the set of MCEE engines, an extension and isolation (EXI) domain in the MCEE logical structure associating at least one node of the tenant resources with at least one node of the first non-tenant cloud and at least one node of the second non-tenant cloud; and connecting, by the set of MCEE engines, for network communications, the E2E-VNS virtual networks of the nodes of the EXI domain for isolation of the resources of the nodes of the EXI domain from the other resources of the MCEE logical structure in an EXI virtual network.