They actually don't, as very few MPs have the necessary legal qualifications. So staff, writes it. It isn't uncommon for legislators to be unable to grasp details of the things they propose.

Unfortunately lobbyists write much of the legislation these days.

Otherwise legal and permanent secretaries ( civil servants ) draft laws and parliaments ( congress ) debate and amend them.

I conceed the executive branch does propose the policy and ideas on which they are based.

I am all for legislatures ( parliaments ) and elected representatives but the executive ( govt. ) seems to increasingly be at odds with the citizenry and serve those who fund their election or pad their retirements with directorships.

Sure have govt. just much less of it - there are far too many laws and far too many bad laws.

Fewer better, well considered laws, informed by public debate.

Really watch a couple of episodes of Yes Minister, that shows how it works not how they say it works.