Every tick the server is running, it iterates through all the loaded chunks in the world that can be ticked (in general, loaded chunks that players are close enough to them) and tries to tick blocks inside these chunks. For each of these chunks, the server iterates through each one of its sections and chooses a number of blocks to try and tick. How does the server choose this number? Game rules! The game rule
randomTickSpeed determines exactly that. It randomly picks up blocks in each chunk section (according to the
randomTickSpeed value) and checks if they can get ticked. If they do, it runs a tick for them. In game, you'll see the block grow.