Compiling custom kernel for supporting nftables kernel firewall

I found that the kernel from the debian-202308 image is not supporting the nftables kernel firewall. So, i want to compile a custom kernel supporting the nftables firewall and routing functionality. As i find the debian-202308 image with a riscv64 architecture, i would like to have a riscv64 custom kernel as well. What is the kernel tag for the kernel used in the debian-202308 image and how do i check it out for local self-compilation and configuration?

I am already contributing to GitHub.