Completely depends on the country. In the UK under the British Judo Association the most common route is competitively.

1st - 5th Dan this consists of either having to get 100 points over a period of time (10 points per ippon), or winning a line-up on the day.

Getting 6th Dan and above is granted by the association for services to judo.

8th Dan and above I think comes from the EJU or the IJF. Someone please correct me.

There is still a requirement to conduct kata and show technical ability which needs to be signed off by a senior examiner.

There is also a non competitive route where technical ability and kata is assessed but this takes longer.

https://www.britishjudo.o[...]5.2016.pdf

Here is the BJA requirement document. I wasn’t far off.

I’m not in the UK, but thanks for the info.

The USJA had a time in grade, point system, kata requirement with exam. It's in a seperate book available on their website. I believe USA Judo and the USJF are similar as well.

I wonder what the French, Mongolian, Korean and Georgian, Uzbek methodologies are. It would be interesting to see what goes in to making a Dan grade for various regions. This would probably also help evolve the training and performance of everyone across the board!