How to Install Windows on a Mac without Bootcamp Assistant?

It is still possible to install Windows 10/8/7 on an Intel based Mac. For dual-boot, you can go with Bootcamp Assistant if it is available on your Mac. If not, then you should first make a Windows bootable USB on Mac and install Windows from the USB drive.

Here is a quick guide you can follow to create a Windows 10 bootable USB on Mac:

https://www.uubyte.com/online-help/iso-editor.html

After that, you should back up your Mac and erase the disk to MS-DOS FAT32 (Master Boot Recorder).

Finally, press the Option key to boot from the drive and start installing Windows on your Mac with Bootcamp.

If you have an Apple Silicon Mac, then things are much more complicated and restricted.

It is possible to use the Windows ARM version with a Parallels' VM, but there are a lot of limitations & restrictions with this option (still requires a proper Windows' license). Both Microsoft and Parallels have some documentation on how this works and the various limitations & restrictions.

Another option would be to use the third party UTM app which uses QEMU at its core which is a free open source machine emulator & virtualizer to mimic different CPU architectures so you can run an OS for a completely different CPU architecture on an Apple Silicon Mac. With UTM you can install the Intel version of Windows OS (you will need a proper Windows' license). However, there are a lot of limitations....the performance will be slow since you are emulating a different system plus running a virtualized instance of an OS as well. Plus you won't have access to any GPU hardware acceleration. For some apps this option may work fine, but for others it will be insufficient.

There is no Windows Bootcamp option for an Apple Silicon Mac so there is no way to run Windows on bare metal on an Apple Silicon Mac.

If it is an Intel-based Mac, you could install Windows inside of a virtual machine and run Windows at the same time that macOS is running.

Parallels sells virtual machine software (Parallels Desktop) – and VMware offers a free personal-use license to use VMware Fusion Player. (Either way, you will need to obtain a legitimate copy of Windows.)

As already stated if you have an Intel Mac you could use a program like Parallels. I use that and have three different Window versions running (XP, 7 and 10) and they all work fine within Parallels.