Not clear. window.electronAPI looks like bridge API registered using something like

contextBridge.exposeInMainWorld('electronAPI', {
    // some methods
})

This implementation is done in preload and can acccess real Electron and node.js APIs and modules. This code creates an IPC API for the renderer code having no priviledge to use those API's directly. The exposing of this kind of IPC API to the renderer's window object is just the implementation detail of you Electron application, even if it was not you who implemented it.

It is unclear, what it is. What exactly is done in the main Elecrton process? Is it Electrom desktopCapturer combined with remote? Something else?

If this is the desktopCapturer, it knows nothing about your renderer's DOM, it captures the image source immediately, 'screen' or 'window'. Or are you doing something else?