Simply run either of:
zig build run-aarch64
zig build run-x86_64-bios
zig build run-x86_64-uefi
to start the kernel in qemu.
The default target
zig build
builds all kernels and images.
To flash a x86_64 universal (both bios and uefi) to a usb drive
zig build x86_64-universal-image
dd if=zig-cache/universal.iso of=/dev/disk/by-id/my-usb-drive bs=1048576
sync