1. Capital controls. Russians are legally not allowed to sell their Rubles anymore. This decreases supply and keeps prices up.
2. A healthy dose of foreign currency ($1B/day from Germany alone) gives the Russian central bank plenty of tools to buy Rubles and increase demand for it to keep prices up.
This does not mean that the existing sanctions aren’t working. Eventually Russian factories will run out of machine spare parts and everything will grind to a halt, people will use their jobs, demand for goods will decrease, other factories will have to close due to lack of demand for their goods, their workers get laid off - you get the idea. This takes time though.
Disclaimer: I’m not an economist, but I do have a subscription to the Economist :)