Subcontracting is the arrangement in which one company asks another to carry out all or part of the work it has agreed with the customer. In the locksmith trade it happens, for example, when a renovation firm, an insurer or a platform asks a local locksmith to change a lock.
It is normal practice in many trades. What matters for you is knowing who is responsible for the work, which is usually the company you agreed the job with and that invoices you.
If someone different from the person you expected is sent, ask who they are working for and check that the price is the one agreed. Communities and property administrators tend to prefer a regular locksmith who knows their buildings, as with lock maintenance.