I'd like to learn and try things out without the need to have access to my Apple device. Currently, I'm regularly on a Windows machine, and I find it a bit inconvenient to switch my work setup/environment; I'd like to have access to Swift and SwiftUI at my fingertips.
On the web, no, AFAIK.
But you can use Swift on windows: https://www.swift.org/blog/swift-on-windows/
But SwiftUI is specific to Apple and require Apple device to run.
In any case, if you want to learn Swift and SwiftUI, that probably means you intend to develop (and possibly publish ?) apps for Mac or iOS or TVOS… Hence, getting a Mac to develop is required.