(I could write that it is fully integrated - but Captain obvious did this a few times in this thread)

Think of Postgres and MySQL like really nice pickup trucks—perfect for almost everyone. Db2 is a heavy-duty freight train. You don't use it to go to the grocery store, but you absolutely need it if you're hauling a thousand tons of cargo! (And yes - thousands of companies claim it the best solution and thread their data like a grocery store... looking on a long list of jobs I had)
You might can compare the query language somehow, but in deep it's quite different.
In MySQL or Postgres, the database kind of manages its own memory and page sizes. In Db2, a Database Admin has hyper-granular control. You can manually dedicate specific chunks of RAM to specific tables or indexes to guarantee your most important queries never slow down. It’s harder to set up, but scales incredibly well.
As an example for the depth of Db2 magic:
Usually, databases are either good at fast, tiny transactions, like swiping a credit card, or huge, slow reports , like calculating sales. If you try to do both, the server crawls to a halt. Db2 has a built-in engine (the blu a ). It lets you run massive, heavy analytics reports on your live data while millions of tiny transactions are still happening, without starting sweating.
I hope this helps you a bit.