Pop OS uses their own implementation named kernelstub to handle the kernels, they did so to make it faster on load times (at least that's what their blog post says).
Their README page show certain information of how to use it, but there are some other things that are required to do, specially if you want to have a menu on boot, similar to grub.
In case you just want to use a specific kernel and forget about other ones, you can execute this command to update the current configuration