I was introduced to this from reading Moonkwalking with Einstein by Joshua Foer, and there's a few communities online you can find with a little searching. Here's an example:
The technique is known as PAO (Person, Action, Object). Each 2 digit number has a unique Person/Action/Object combo. At the time wrote my own PAO I was entrenched in a foreign language that puts the verb at the end of the sentence, so I forever think Person/Object/Action for this system now but that's not a big deal, you could conceivably use any order you want as long as you're consistent.
An example: the number 23 makes me think of Michael Jordan, so I assigned Michael Jordan/Basketball/Slam Dunk to the number 23. 84 is Winston Smith/Pen/Write in a Diary. 01 is Frodo/the One Ring/Become invisible.
To remember the number 842301 you parse the number into 2 digit strings 84/23/01 and you replace them with your POA to create a memorable mnemonic. Winston Smith putting a basketball on his finger and turning invisible. This one mnemonic is information dense, it has 6 digits. Create 17 of these and put them in your memory palace and you've memorized the first 102 digits of pi.
To memorize populations of countries I replaced the Person part of the mnemonic with some person from that country. For Russia it was Putin, for Mexico it was a Mexican friend of mine. My goal was to remember how many millions of people; the only exceptions on the list were China and India which had billions but that was easy enough to remember without a uniform mnemonic.
Russia has 146 million people. Putin/1/46. My number 01 is Frodo, 46 is the Hulk. Putin using the one ring to "hulk smash" St. Basil's Cathedral. Because the memory palace is ordinal it was also easy for me to rattle off things like "Russia's the 9th most populated country in the world".