At the outset, I would restrict my reply to the query with reference to the term "worker" as defined under section 2(l) of the Factories Act, 1948. For the purpose of interpreting the term, one should read it together with the definitions of "factory" and "manufacturing process." All those employed directly or indirectly in any manufacturing process or any other kind of work incidental to or connected with the manufacturing process or the subject of the manufacturing process are workers. If the administrative or other office is situated within the premises where a manufacturing process is being carried on, the entire area will be treated as a factory, and all the people employed therein would be included in the numerical criterion required for the Act's application. Salary/wages are not determinants in this connection. However, if any of them have to perform their work normally outside the premises, they will not be workers under the Act for its purpose. Therefore, the 20 people directly engaged in the manufacturing process and all others doing their work within the premises like accounts, HR, IT, design, watch and ward are workers under the Factories Act.
Since the post has started with a blanket question, I would also like to add that the application of labor law in general would depend upon the purpose of a particular law, salary/wage, and the capacity/status of employment of the employees. For example, any employee irrespective of their employment capacity/status and salary would be eligible for gratuity under the Payment of Gratuity Act, 1972, if it applies to the establishment. The Industrial Disputes Act, 1947 would apply to all employees who fall within the ambit of the definition of the term "workman" for the purpose of protection of the employment rights ensured therein.