You are extremely technical or cpio is extremely technical. Or both,I don't know. For sure I'm a hobbyist and I have trouble understanding some technical concepts. So,I'm not sure that I have understood. But I tried to follow your directions,issuing the commands below. But I've got the same error as before :
https://ibb.co/rm5WRSz
mkdir /home/ziomario/Scrivania/PassT-Cubic/kernels
mkdir /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped
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
mkdir -p usr/share/plymouth/
mkdir -p usr/share/plymouth/themes/homeworld/
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
cpio -idvm < initrd.img-5.10.0-18-amd64 -D /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64
cpio -idvm < initrd.img-5.10.0-19-amd64 -D /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64
cpio -idvm < 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
cp -p usr/share/plymouth/debian-logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-18-amd64/usr/share/plymouth/
cp -p 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 -p 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 -p usr/share/plymouth/debian-logo.png /home/ziomario/Scrivania/PassT-Cubic/kernels/unzipped/initrd.img-5.10.0-19-amd64/usr/share/plymouth/
cp -p 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 -p 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 -p 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 -p 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 -p 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 . -print -depth | cpio -o > ../../initrd.img-5.10.0-19-amd64
find . -print -depth | cpio -o > ../../initrd.img-5.19.0-15.2-liquorix-amd64
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
Mario.