Dolphin can not be used as file picker.
All apps that use the native installed Qt of your distribution will use the KDE file picker on plasma. Flatpak apps are usually using xdg-desktop-portal and therefore also use the KDE file picker automatically. Apps shipping their own Qt or using GTK outside of flatpak / snap can not easily use the KDE file picker. You force GTK apps to act as if they were running in a sandbox using the GTK_USE_PORTAL=1 environment variable, which should also result in a KDE file picker.