Deferred entires?

I've recently started thinking about deffered entry, and was wondering the process?

1. Does it involve applying with official grades or predicted grades? Or is it the same thing of apply with predicted and hit that mark on results day?
2. When do you apply for it?
3. Do you have to have work experience lined up in the year between Uni?
4. Is it related to clearing?

I appreciate any answers!

you apply for deferred entry as if you were applying for direct entry and at the same time - you just begin the course a year later. You use your predicted grades if you are still in school (or achieved grades if you have finished school already). If applying with predicted grades any offers will be conditional on achieving those grades that year.
So for example if you are currently in year 13, you would apply by october 2025 deadline for oxbridge/med/dent/vet or january 2026 deadline for other universities/courses, for deferred entry to begin in fall of 2027, with predicted grades. You will need to achieve the conditions of your offer by summer 2026 in order for your deferred offer to become unconditional. If you fail to achieve those conditions you do not get an "extra year" to meet those requirements - you would be rejected and have to reapply during a gap year if you wished to.
Whether to arrange work experience or anything else in the gap year is up to you. The university has will impose no requirements in that sense.
There is no relation to clearing - and clearing is a separate process for entry for the coming academic year, not for deferred entries (as it's a system designed for unis to fill places for the upcoming year).