To install .deb files in an Arch-based Linux distribution like Manjaro, you can use a tool called debtap. debtap allows you to convert .deb files to Arch-compatible packages that can be installed with the package manager.
I have a youtube tutorial for this process. To watch, click here

Here are the steps to install and use debtap:

1. Install debtap using Pacman, the package manager for Arch-based distributions. Open a terminal and run the following command:

   sudo pacman -S debtap
   

2. Once debtap is installed, you need to update its database with the command:

   sudo debtap -u
   

3. To convert a .deb file to an Arch-compatible package, navigate to the directory where the .deb file is located in the terminal, and run the following command:

   sudo debtap -v .deb
   

   Replace with the name of the .deb file you want to install.

4. debtap will convert the .deb file to an Arch-compatible package and save it in the current directory. To install the package, use the Pacman command:

   sudo pacman -U .pkg.tar.zst
   

   Replace with the name of the package generated by debtap.

5. The package will be installed on your system, and you can use it like any other package installed through the package manager.

NOTE: that debtap does not guarantee that the converted packages will work perfectly, as they may have dependencies that are not available in the Arch repositories. You should also exercise caution when installing packages from sources other than the official repositories, as they may contain malicious code.