Systemwide uniform keyboard remapping in Gnome DE to match Mac keyboard shortcuts?

You're asking about two different kinds of shortcuts, which are implemented in different places.

Things like switching between apps or windows are handled by a central component – the window manager – in practically all operating systems. As the window manager ultimately decides the position and stacking order of all windows, it's also where shortcuts like Alt-Tab (switching between windows) or Alt-F4 (closing a window) are implemented as well, allowing them to be globally configurable.

(At least these two particular shortcuts are shared with Windows and the old IBM CUA standard.)

On the other hand, things like copy/paste or text cursor movement must be implemented by the app, as only the app knows what kind of data it's dealing with, and what's currently selected. Most commonly, however, apps are built using an existing UI toolkit such as GTK or Qt which takes care of implementing copy/paste for its standard widgets. This is similar to AppKit or UIKit in macOS.

Among those toolkits, Qt has deliberate integration with the global KDE settings, being able to load various toolkit-level keybindings from global configuration (allowing you to change e.g. the copy/paste keys in theory), but other toolkits generally don't do that. For example, there are no global GNOME settings that a GTK app would use, and it will not care about KDE's either.

(Alongside the traditional ones, it's worth mentioning the issues with cross-platform toolkits which try to adapt to multiple operating systems with varying levels of success – such as Electron, the "web browser pretending it's an app" toolkit that e.g. VSCode uses on both Linux and macOS. Similarly, LibreOffice runs on several operating systems and uses its "VCL" toolkit that can masquerade as either GTK or Qt or as the Windows UI. Web browsers such as Firefox and Chrome also use their own UI toolkits, they're just making a bit more effort at adapting to each individual OS.)

The concept of "preferences" or "settings" is also app-specific – each app has its own way to store settings and its own dialogs; it's not something provided by the OS. The difference is simply that macOS has a bit of stronger push towards its standard HIG guidelines (e.g. Ctrl-, and the "Preferences" terminology) so that developers are more likely to agree on the same set of shortcuts. (GNOME has been slowly improving on this front, adopting a somewhat macOS-inspired style; I'm not familiar with what KDE is like these days.)

...and of course macOS has one desktop environment with one set of guidelines, whereas in Linux you have at least two major environments (GNOME and KDE) with sometimes conflicting ideals. Running something like Kate (the main KDE editor) in a GNOME environment, or vice versa, will always feel a little out-of-place. Not to mention the various Electron-based apps like VSCode.

To quit an app on Linux, sometimes it's Ctrl-q, sometimes Alt-F4

Alt-F4 is implemented as a window manager shortcut (most WMs using it by convention), so it's configurable globally and can operate on any window regardless of the app. However, it's not a "quit app" shortcut, it really just closes the single window.

Ctrl-Q, on the other hand, is an app-specific shortcut.

To jump to the beginning or end of a line you could use Home / End on single-line text fields, but I haven't found a way to do this in multi-line text editors.

While each toolkit (and especially the custom widgets within apps) might have different shortcuts, I haven't seen one that wouldn't support Home/End – it's pretty much universal. To jump to the start of the first line, or to the end of the last line, the shortcut is Ctrl-Home/End (just like in Windows).