It seems that OP does not want to use the PPA, which can be found in N0rbert's answer. There is a way to directly generate the .deb package from the source without using the PPA.

1. Install the build dependencies.

   
   

   sudo apt-get update
   
   sudo apt-get install git build-essential appstream debhelper desktop-file-utils libgranite-dev libdazzle-1.0-dev meson libgtk-3-dev python3 python3-pip python3-setuptools python3-distro python3-requests python3-tz python3-gst-1.0 python3-cairo python3-apsw sed
   
   

   
   

   Ubuntu 20.04 LTS does not provide libhandy-1, so we need to build it locally using sources from Ubuntu 21.04:

   
   

   sudo apt-get install debhelper-compat gtk-doc-tools libgirepository1.0-dev libgladeui-dev libglib2.0-doc libgnome-desktop-3-dev libgtk-3-doc libxml2-utils valac xvfb
   
   cd ~/Downloads
   
   wget -c http://archive.ubuntu.com/ubuntu/pool/main/libh/libhandy-1/libhandy-1_1.2.0.orig.tar.gz
   
   tar -xf libhandy-1_1.2.0.orig.tar.gz
   
   cd libhandy-1-1.2.0
   
   wget -c http://archive.ubuntu.com/ubuntu/pool/main/libh/libhandy-1/libhandy-1_1.2.0-1.debian.tar.xz
   
   tar -xf libhandy-1_1.2.0-1.debian.tar.xz
   
   rm libhandy-1_1.2.0-1.debian.tar.xz
   
   
   
   dpkg-buildpackage -uc -us -d
   
   sudo apt-get install ../gir1.2-handy-1_1.2.0-1_amd64.deb ../libhandy-1-0_1.2.0-1_amd64.deb ../libhandy-1-dev_1.2.0-1_amd64.deb
   
   

   
   



2. In Ubuntu 20.04, manually download a recent version of python3-peewee and python3-mutagen (in later versions of Ubuntu, you can install it from the repositories.),

   
   

   cd ~/Downloads
   
   wget -c http://old-releases.ubuntu.com/ubuntu/pool/universe/p/peewee/python3-peewee_3.13.1+dfsg-1_amd64.deb
   
   wget -c http://archive.ubuntu.com/ubuntu/pool/universe/m/mutagen/python3-mutagen_1.45.1-2_all.deb
   
   

   
   

   Install them

   
   

   sudo apt-get install ./python3-peewee_3.13.1+dfsg-1_amd64.deb
   
   sudo apt-get install ./python3-mutagen_1.45.1-2_all.deb
   
   

   
   



3. Install all the runtime dependencies

   
   

   sudo apt-get install gobject-introspection python3 python3-distro python3-gi-cairo python3-requests python3-packaging python3-tz python3-mutagen python3-gst-1.0 python3-gi python3-cairo python3-apsw file libgstreamer1.0-0 libgstreamer-plugins-base1.0-0 gstreamer1.0-plugins-good gstreamer1.0-plugins-ugly gstreamer1.0-plugins-bad gstreamer1.0-libav gir1.2-granite-1.0 gir1.2-dazzle-1.0
   
   

   
   



4. Download the source from GitHub:

   
   

   cd ~/Downloads
   
   git clone https://github.com/geigi/cozy.git -b 1.2.0
   
   

   
   



5. Download the debian/ directory from http://github.com/apandada1/cozy/tree/deb-dir and place it inside the source directory of cozy.

   
   

   cd ~/Downloads
   
   git clone https://github.com/apandada1/cozy -b deb-dir debian
   
   mv debian/debian cozy/
   
   

   
   



6. Now build the .deb package and install it with

   
   

   cd ~/Downloads/cozy
   
   dpkg-buildpackage -uc -us -b
   
   
   
   sudo apt-get install ../cozy_1.2.0_all.deb