According to Substrate doc on Runtime development: "...the runtime for a Substrate node contains all of the business logic for executing transactions, saving state transitions, and interacting with the outer node."
Can anyone please point out where the APIs are for these functions? ("executing transactions, saving state transitions, and interacting with the outer node.") Is it "sp_api"? (https://docs.substrate.io/main-docs/fundamentals/runtime-intro/)
My understanding is that we don't need FRAME and Pallets to build a runtime, and building one from scratch probably is necessary to better understand how Substrate works under the hood.
Is there an example how this can be done?