One product, several platforms. Each has already taught your users how it works, and nobody asked you. Multi-platform design is deciding what stays fixed and what is free to differ.
Fixed: concepts, terminology, information architecture, brand, what the thing is for. Colour, voice and vocabulary get no per-platform variant — if it is called Collections on the web, it is Collections on Android and holds the same items. Free to differ: navigation chrome, back behaviour, date entry, sharing, permission prompts, gestures, keyboard handling. Anything the operating system owns, let it own.
Both failure directions are expensive. Port a design pixel for pixel and it feels foreign — a hamburger where a tab bar belongs, a web app whose screens have no URLs because the phone version needed none. Let each platform team design freely and the same feature acquires a different name and shape on each, and support answers every question twice. A design system carries part of this load, but only part: tokens travel, components do not.