What can I do to fix or replace this broken axle? I cased a jump badly and I broke either my bottom bracket or crank, I’m not sure which. What can I do to fix or replace this?

Picture is of my broken bottom bracket or crankset.
You've broken your bottom bracket's axle.
There is no way to safely reattach, so you're up for a replacement bottom bracket.  Ideally you'd install a cartridge BB that duplicates what you have.
The crank arm (in your hand)  is probably okay to reuse, though have a good clean and a close inspection before doing so.  Dings are fine, chips maybe, and cracks are not.  You need a crank puller to get the sheared-off stub out of the crank arm.
I'm not sure if your bike uses a square taper or one of the newer standards - the broken bit could be square or round.  So your new BB has to match what the cranks fit onto, otherwise you'd need new cranks too and that's getting expensive.
Tools should be mostly common hand tools (ie a big spanner), and you will need a BB tool to fit the splines visible in your photo, and a crank extractor to push the nubbin out of the crank arm.   That last might be possible using a press or a hammer and brass drift, but only because its already off the bike.

A follow-on thought - to break a bike like this means either a very hard impact or a lack of maintenance on your part.
Based on the corrosion I can see in the photo, there's a part which was already cracked and exposed to the elements for long enough for rust to start.
Upshot - there was a fairly hard impact, and this was focused on pre-existing damage that you hadn't noticed.  I'd strongly recommend a good clean and de-grease of your entire bike, and go over it looking for other looming damage.  Or ask someone else to help with this - its easy to miss things.