I tried to follow your directions,using cp usr/share/plymouth/debian-logo.png instead of cp /usr/share/plymouth/debian-logo.png. I hope that this is what you intend. So,below there are the commands that I have issued :
cd /home/ziomario/Scrivania/PassT-Cubic/kernels/
gunzip -k initrd.img-5.10.0-18-amd64.gz
gunzip -k initrd.img-5.10.0-19-amd64.gz
gunzip -k initrd.img-5.19.0-15.2-liquorix-amd64.gz
mkdir /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64
mkdir /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64
mkdir /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64
cpio -idv < initrd.img-5.10.0-18-amd64 -D /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64
cpio -idv < initrd.img-5.10.0-19-amd64 -D /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64
cpio -idv < initrd.img-5.19.0-15.2-liquorix-amd64 -D /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64
rm /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64/usr/share/plymouth/debian-logo*.png
rm /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64/usr/share/plymouth/debian-logo*.png
rm /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64/usr/share/plymouth/debian-logo*.png
cp usr/share/plymouth/debian-logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64/usr/share/plymouth/
cp usr/share/plymouth/themes/homeworld/debian.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64/usr/share/plymouth/themes/homeworld
cp usr/share/plymouth/themes/homeworld/logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64/usr/share/plymouth/themes/homeworld
cp usr/share/plymouth/debian-logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64/usr/share/plymouth/
cp usr/share/plymouth/themes/homeworld/debian.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64/usr/share/plymouth/themes/homeworld
cp usr/share/plymouth/themes/homeworld/logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64/usr/share/plymouth/themes/homeworld
cp usr/share/plymouth/debian-logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64/usr/share/plymouth/
cp usr/share/plymouth/themes/homeworld/debian.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64/usr/share/plymouth/themes/homeworld
cp usr/share/plymouth/themes/homeworld/logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64/usr/share/plymouth/themes/homeworld
mv initrd.img-5.10.0-18-amd64.gz initrd.img-5.10.0-18-amd64.gz-old
mv initrd.img-5.10.0-19-amd64.gz initrd.img-5.10.0-19-amd64.gz-old
mv initrd.img-5.19.0-15.2-liquorix-amd64 initrd.img-5.19.0-15.2-liquorix-amd64-old
cd /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64
cd /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64
cd /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.19.0-15.2-liquorix-amd64
find . -print -depth | cpio -o > ../../initrd.img-5.10.0-18-amd64
find: warning: you have specified the global option -depth after the argument -print, but global options are not positional, i.e., -depth affects tests specified before it as well as those specified after it. Please specify global options before other arguments.
893621 blocks
find . -print -depth | cpio -o > ../../initrd.img-5.10.0-19-amd64
find: warning: you have specified the global option -depth after the argument -print, but global options are not positional, i.e., -depth affects tests specified before it as well as those specified after it. Please specify global options before other arguments.
893621 blocks
find . -print -depth | cpio -o > ../../initrd.img-5.19.0-15.2-liquorix-amd64
find: warning: you have specified the global option -depth after the argument -print, but global options are not positional, i.e., -depth affects tests specified before it as well as those specified after it. Please specify global options before other arguments.
893621 blocks
cd ../..
gzip initrd.img-5.10.0-18-amd64
gzip initrd.img-5.10.0-19-amd64
gzip initrd.img-5.19.0-15.2-liquorix-amd64
no. Unfortunately the produced kernel files are not able to boot. In Fact the size is bigger than the original ones. This is the error reported :
I don't know why. Inside the kernel files It seems that everything is ok. I have placed the wrong files in my google drive. Maybe you want to test them on your side ? Thanks for your very very useful support. I can tell for sure that the quality and your patience are the best that I found on the internet.
Mario.