I think there’s enough anecdotal and research evidence to show that some parrots do understand what they are saying to the extent that they associate a phrase with a meaning (like someone ignorant of German saying “Gesundheit!” when someone sneezes.)
Bird brains have been naturally selected, as avionics have been deliberately designed, to be small, light and low energy. My completely incorrect and misleading analogy would be that if mammalian brains are X86 and occupy desktops, bird brains are ARM and occupy mobile phones. Which is why you cannot compare the size of bird brains to mammalian brains to get some idea of comparative “intelligence”.
