As a mobile dev, it should be the same for mobile apps?
If you are on iOS, you could just use what Apple gives you with very little customizations. The app would feel right, dark mode, font sizes, screen traders, etc. accessibility would "just work" 98% of the time, the app would be fast.
On Android, things are a tad bit more complicated, but it still applies that the closer you are to the OS, the better things work out of the box.
But for some reason, companies, their designers, product owners, developers want to get creative, use their brand but eventually they always run out of time to implement things properly, so apps are slow, user's font scale settings are not respected, dark mode is a feature, and the app doesn't feel like part of the same phone / OS.
I strongly believe that the amount of ironing they have to do for that crossplatform toolkit takes more time than just going with a common core and different UI shells.
If you are on iOS, you could just use what Apple gives you with very little customizations. The app would feel right, dark mode, font sizes, screen traders, etc. accessibility would "just work" 98% of the time, the app would be fast.
On Android, things are a tad bit more complicated, but it still applies that the closer you are to the OS, the better things work out of the box.
But for some reason, companies, their designers, product owners, developers want to get creative, use their brand but eventually they always run out of time to implement things properly, so apps are slow, user's font scale settings are not respected, dark mode is a feature, and the app doesn't feel like part of the same phone / OS.