NOTE!! The @stencil/core/internal package is not meant to be consumed directly by anything other than Stencil internals. It is its own package so that it can be resolved by Stencil, but breaking changes can/will happen at any time. This isn't a "use at your own discretion" moment, but it's more of a "never use this because your code will break" fact.
This is the main entry file for all of Stencil's internals, such as Stencil's runtime and compiler types. However, any public references to the internals are found in declarations/stencil-core.ts and internal/default.ts. This file is largely used to generate all of Stencil's internal types. But the transpiled JavaScript from this is not exposed.
By default, when Stencil resolves @stencil/core/internal, it's going to assume it wants the client internals (rather than hydrate). So by default, @stencil/core/internal actually points to @stencil/core/internal/client.