I saw online that metal is only compatible with Windows 10. I just joined the Apple development program to start building apps for the firs time but I don't see the point if my apps won't be compatible on all platforms. Does anyone know Apple's plans regarding Windows 11?
Low-level graphics user interfaces have long involved platform-specific APIs, or higher-level and more portable interfaces. The former tend to be faster, more efficient, and more capable. The latter, well, the latter tend to be higher-level APIs.
Some folks will partition their app designs into hunks, with platform-specific code isolated from platform independent code. Graphics APIs tending to be platform-specific. Sometimes different UIs for the same platform, for some apps.
For more portable graphics APIs for apps, Vulkan is going to be interesting to you (MoltenVK with Metal, etc), or a language-specific framework.
Maybe using Unity or Unreal Engine, depending on your targets and apps.
As for Microsoft Windows 11, Apple doesn't usually post their future plans, and—Microsoft announcements and PR and press aside—Windows 11 has yet to be released. And much like Apple and Metal, Microsoft has their own preferred graphics interfaces for the folks writing apps for that platform; DirectX, etc.
There's no good answer here, and different apps and different approaches have different trade-offs.