The Linux kernel, as well as the FreeBSD kernel, supports the loading and unloading of kernel modules, which are compiled binaries that can be linked to the kernel while it is running. These modules are still in the address space of the kernel; the term "monolithic" refers to the fact that all portions of the kernel, even kernel modules, are running in the same address space. This is different from a microkernel like Mach where various operating system subsystems such as networking and the file system are running as separate user-level processes (thus having their own address spaces).

"Monolithic" refers more to the way drivers and the kernel interact, i.e. if drivers live in the kernel address space, can read/write all kernel memory, call functions in other drivers, etc., then it's a monolithic kernel. If instead each driver is its own process with address space, and calls into the kernel via syscalls like any other process, then it is a microkernel.

"Modular" refers to linkage and overall design. The Linux kernel is mostly "statically" linked (there is DKMS but it isn't so widely used) with most major systems in the "kernel" itself, and something similar is true in the FreeBSD world.

Haiku, on the other hand, has a rather small (2.2MB binary on i386) kernel with no drivers built in at all (not even PCI); and loads all its drivers as relocateable shared-objects. This means unloading and reloading kernel modules at runtime (or boot) is very easy; so as long as you don't crash or hang the system, you can keep reloading drivers as much as you like without rebooting.

I know macOS also uses dynamic kernel modules, but I'm not as familiar with that system.