how can i download ubuntu's kernel?
You can query them in their packages search list. https://packages.ubuntu.com/search?keywords=linux-image-generic
This is for Ubuntu 21.10: Package: linux-image-generic (5.13.0.21.32 and others)
tar.xz: http://archive.ubuntu.com/ubuntu/pool/main/l/linux-meta/linux-meta_5.13.0.21.32.tar.xz