A component can be a product in its own right Art. 3(1), Recital 18
The CRA defines a product with digital elements to include software or hardware components placed on the market separately, and it defines a component as software or hardware intended for integration into an electronic information system. A library, SDK, or module you distribute for others to build on can therefore be a product with digital elements when placed on the market on its own.
The commercial-activity test still decides scope. For components intended for integration by other manufacturers, the recitals say supply counts as making available on the market only if the original manufacturer monetises the component. An unmonetised open-source component you publish is treated differently from one you sell or license commercially.
If you integrate third-party components Art. 13(5), Art. 13(6), Annex I Part II
The other side of libraries is consuming them. When you integrate components sourced from third parties into your product, you must exercise due diligence so those components do not compromise your product's cybersecurity, including for open-source components not made available on the market in a commercial activity.
Your vulnerability handling covers the product in its entirety, including all integrated components. On identifying a vulnerability in a component, you must report it to whoever manufactures or maintains the component, remediate it in your product, and, where you developed a fix, share the relevant code or documentation with them.
Information for integrators Annex II (8)(f)
If you supply a component intended for integration into other products with digital elements, the information accompanying it must include what the integrator needs to comply with the essential cybersecurity requirements and the documentation requirements. That makes the interface between component supplier and integrator part of the compliance chain, not an afterthought.
Practically, that means documenting security properties, intended use, and the information an integrator needs to meet Annex I, so your customers can carry their own obligations. Where the CRA text is thin on a specific component scenario, treat the gap as a point to reason through and record.
Judgment call: The CRA sets the integrator-information duty but leaves the exact contents to the manufacturer's judgment for a given component; document what an integrator would reasonably need.