In fact the input field is one of the few native UI surfaces in the Telegram app.
it's not a native widget. you can type styles on whatsapp which is not possible on native input.
whatsapp is the poster child of this technique btw.
Styles are possible with native text view, otherwise we couldn’t have fonts and colors and contenteditable and selection within <p>. I think you mistake “input core” with “web input element” here. The core I’m talking about is a text cell with characters on it that has no concept of own border. Controls like NSTextField, Win32.TextBox, GtkEntry - they all have platform text input (and output) system underneath. In case of gtk it implements it itself, cause X has none. The thing that implements all the familiar behaviors of the text input that you know.
So yes, compared to HTMLInputElement these are absolutely custom. But for a platform, absolutely native.
Too lazy to check on android rn, but I recently worked with the apps/chats on it and entered text, it didn’t feel different.