The schemas can be found in the native assets directory as specified here: https://www.itwinjs.org/reference/core-backend/imodelhost/knownlocations/

The user then needs to know that schemas are in ECSchemas/Standard sub directory.

Consider making a locater that can find these schemas and can be added to any context.
Consider also having a locater that can locate the delivered domain schemas (like BisCore). This may not be directly possible due to the fact that core-backend does not depend on ecschema-metadata and ecschema-locaters should not depend on core-backend.

BisCore and other domain schemas return their path via a property on the class for that schema (e.g. BisCoreSchema). So long as it's easy to load them given the full paths that may be enough

Locating these schemas should be opt in or lowest priority so any location specified by the user takes precedence.