First day for new Minister - s/he gets a briefing on all the current issues from the top Sir Humphreys in the Dept. They spell out the pros and cons of whatever policy issues need sorting and make a recommendation eg 'Minister, can I recommend that you choose this option to solve this problem'.
Ministers also have Special Advisors (2 each for a Cabinet Minister usually) - these are party political helpers who make it their business to help the Minister get on top of the new area. You can be sure the SpADs of the outgoing Minister have had a chat with the SpAds of the incoming Minister so they have an idea of what's what.
A wise new Minister holds off saying yeay or nay to anything controversial until they're sure they understand the arguments.
After a while the Minister gets on top of the new subject area. Then they start arguing with the civil servants' recommendations ... The civil servants then look forward to the next reshuffle, when they might get someone less argumentative sent to the Dept!
The other thing to say is that most senior politicians on all sides have quite a good broad brush working knowledge in a lot of policy areas because they've been in politics a long time. It's usually only the detail they need to gen up on.