You could try something simple like kefir, we did it for our dog mostly. It is very simple. You just strain the yoghurt when it's at your desired thickness, then put fresh milk and the strained grains back in the jar. Over time you'll get too many grains, so you can be the annoying person giving them away and converting people or just feed them to the dog like we did. Optionally, you can do a second ferment in the fridge to let it mature, with optional flavourings. We quickly had too much to use but it works as a substitute for buttermilk and yoghurt in recipes.
I got the original grains from Facebook marketplace or something, we met in a carpark and she sold me a jar for $10 and the whole thing felt a bit like a drug deal. Just like transplanting a plant, you might have middling results until the organisms get used to your way of doing things (your water and room temperature, how long you wait, milk ratio, etc).
Then I forgot about it for a while and it all died but it was very obvious when it "turned", and took a lot longer than expected.
I did kombucha in a 5 gallon bucket with a spigot, and by the time my spouse got tired of the smell in the laundry room / pantry, the scoby was 4" thick. My chickens loved it when i tossed it to them.
I also managed to give some scoby away, to a couple friends and the person who invented the nicotine patch - so that part is also apparently common!
I got the original grains from Facebook marketplace or something, we met in a carpark and she sold me a jar for $10 and the whole thing felt a bit like a drug deal. Just like transplanting a plant, you might have middling results until the organisms get used to your way of doing things (your water and room temperature, how long you wait, milk ratio, etc).
Then I forgot about it for a while and it all died but it was very obvious when it "turned", and took a lot longer than expected.