shooting in 1080/24p - exporting as PAL

Both 1080i50 and 108024P use the exact same pixel dimensions so there should be no difference there.

If you are making regular DVDs it all gets converted to standard def anyway.
99% of PAL DVD players will handle a NTSC disc without issues, as it is.

If you only need a couple of copies for one or two people, that is the simple option.

If you are making thousands of copies with no control over who the end users will be or what device they use for playback, then you should be looking at having a PAL master made with a hardware based standards converter.

how does FCP turn a 24fps clip into 25fpc?

You should not do this in FCP. Edit your 24p project and export it as it is.

Then you can take that master file, either to a facility that offers real time hardware based standards conversion, or do it in software -which will take quite a while.

Software standards converters include JES Deinterlacer (free) or Natress Standards Conversion ($100). JES will do a decent job but in the end, you get what you pay for.

Compressor will do it too, but it takes ages and I think the result is not really worth it. If you do choose to convert, have it done properly by folks who know what they are doing. It's not a good stage in the workflow to start trying to cut costs.