Boot Camp
Does switching between Mac and Windows in Boot Camp always require a restart?
If so is there another piece of software that can do this without restarting?
iMac 27″, macOS 10.15
Yes. That is the advantage of Boot Camp installed Windows. Windows and the MacOS are installed natively and can not run simultaneously. This allows each operating system to use all the system resources when booted.
You can install Windows using a virtual machine like Parallels, Fusion, or VirtualBox. This allows running Windows and your MacOS simultaneously. The problem is that both operating systems share your system resources so neither can run its best.