Being paid to do something does not automatically make all that is involved in the doing of that thing property of they payee.
If I am being paid to produce some deliverable product or deliverable service, then the infrastructure used to create that may very much not be a deliverable or property of the customer.
My point isn't "you are wrong", but rather, there is not necessarily an automatic assignment of such intermediate products. And depending on the type of work (and/or customers / employers you're working for), the answer may vary. Law, custom, employment contracts, and/or engagement contracts will almost certainly play a role here.
If that governing rule is "you shall maintain a project journal and that journal is considered work product and property of the employer", then yes, that is part of what you're being paid to do. Though this may still give you rights to the contents of that journal.