In a parliamentary democracy, every legislative decision is made democratically by the parliament, unless they made the decision to delegate it.
For the topic of immigration policy, there are the following options:
- The parliament votes on a case-by-case basis for every single immigrant. This might work for a small city-state, but it would be impractical for any larger nation which admits more than a few migrants per year.
The parliament votes to delegate the decision in the individual cases to the executive branch, but also makes a law which provides a very objective and stringent ruleset which tells them how to decide.
Expert opinions are considered while the law gets written. It is general practice in many parliaments to write laws by forming a committe which consist of both elected representatives and unelected experts appointed by the elected representatives. The purpose of such a committee is to have the representatives provide political goals ("improve our workforce", "respect our commitment to human rights", "protect our culture and national identity", etc.) and have the experts suggest what measures would achieve those objectives. But the final vote on the law is left to the representatives only.
The parliament votes to delegate the decision in the individual cases to the executive branch, but only gives them very lose guidelines how to decide or even none at all. So the executive branch has permission to set their own rules as they see fit or even leave the the individual decision to the personal judgement of low-level officials.
In this case, the executive branch is expected to hire experts to come up with their rules and/or make case-by-case decisions.
When it comes to immigration, most parliaments decide on an option which is somewhere between 2 and 3. On the one hand, they do not want to lose control over immigration policy. But on the other hand, delegating the details to the bureau of immigration can be more efficient than arguing about them between politicians. Delegating details to the executive branch can also absolve the representatives from responsibility ("no, WE didn't make the decision to reject those political refugees who then got executed by their government. The Bureau of Immigration did that. And those they did accept who turned out to be criminals? Also the bureaus fault, not ours.").