Whether it's an established lawn or a newly seeded lawn. If it's got creeping grasses in the seed, it can be thickened up with ease. All it needs is a little time and to follow these steps.
In this video I apply a quarter rate high Nitrogen fertiliser (46-0-0) - any will do. This one will be fine https://amzn.to/3XkfeSL
I also apply some seaweed (Multi-mite - https://amzn.to/3XjFYmw)
I also apply some Emerald Green (my own product - https://lawnright.co.uk/product/emerald-green/)
Then water it in..
Finally mowing almost daily!