'Always on top' window on macOS
Try Deskovery3. It does that and more. (up to and including Catalina)

There's also Afloat (up to Mojave): http://www.perfectlyrandom.org/2016/10/23/always-on-top-in-macos-sierra/

Or AfloatX (for Catalina): https://github.com/jslegendre/AfloatX

Note: these require you to temporarily lower the security settings on your Mac (disabling SIP) during part of the installation/setup.

Alternatively, you might want to investigate the following macOS features: Full Screen, Mission Control, Hot Corners and Spaces which all allow easy and flexible control of application windows.