Most of the lines of code within the Linux project are about device drivers. Its not about about 1000 lines of code, nor 1 million lines of code but device drivers need around 20 million lines of code within the Kernel. Instead of analyzing what the drivers are doing from a technical perspective there is a need to describe the philosophy.
In classical closed source operaring systems, the device driver is provided by the hardware manufactorer. A certain company is producing a flatbad scanner and has to deliver the hardware itself plus a 3.5" floppy disc which contains of drivers to run the hardware. The same issue is there for a mouse, an usb stick, a camera and so on. In the 1990s it was common that device drivers where delivered on physical discs inside the box of the hardware. The end user was asked to insert the disc into the pc and run a program which was mostly setup.exe to install the drivers. Then and only then the hardware was working.
More recent version of Windows are installing the needed drivers in the background without human interention. The Windows operating system detects with plug and play which hardware is in use and downloads the drivers from the internet. These drivers are mostly writtein in C language and compiled into executable binary programs.
In contrast the linux kernel is working with open source hardware drivers. The Kernel is basically a collection of drivers for getting access to all the devices like cd-rom, ssd, ethernet card and so on. The shared similarity between Windows and Linux is that somebody has write all the drivers. Within the Windows ecodsystem this task is handled decentralized. Each company has to write its own drivers and doesn't explain to the public what the code is about. In contrast, Linux is working with a centralized model. There is only a single kernel and all the drivers are in the kernel.
The focus on the device drivers might explain why apart from the major three operating systems (win, linux and macos) there are no alternative projects available for desktop PC. Everybody who likes to establish a new operating system has to make sure that all the hardware is working with this operating system. The only way for doing so is to write all the needed drivers from scratch. This will take a lot o man years. Because of this single reasson there is no Forth operating system, and smaller projects like Haiku are not working well enough for production machines. The cause is, that most devices won't work with these devices. That means, the proud user of the Haiku OS is plugging in an USB stick into the PC but nothing happens. The OS isn't detecting the hardware and has no executable driver for this hardware.
The major cause why device drivers are released as closed source for Windows is because it is a time consuming task to write the software. A single expert programmer is able to create around 10 lines of code per day. Even if the programmer has access to all the hardware specification and has a lots of experience he will need months up to years until the driver for a certain device was written. It doesn't make sense for a hardware company to release the software as open source because the source code including the ability to write code for new devices is a asset for a company which can't be shared with other comapnies.
The Linux ecosystem is working the opposite way. Its a mandatory rule that all the code has to be released in the open source version. If a certain driver is not available then the device won't work with the kernel. The result is, that the quality of the hardware drivers in Linux is lower and that the amount of drivers is lower. There are lots of hardware available which is supported in Windows but not in Linux. Its not of technical requirements but because of the ecosystem and especially the time consuming effort to write a driver in C.
Suppose it would be possible to create a universal device driver in 10k lines of code which can interact with any possible hardware. Then it would be pretty easy to create new operating systems from scratch. All what is needed is this single device driver and some additional programs can be added. Unfurtunately there are technical limitations which prevents that such a universal driver can be realized. Existing computing hardware is so complex and so different that any single device will need a dedicated driver.
Let us estimate how many different hardware devices are available. suppose a single device like an ethernet card is controlled by driver with 10k lines of code. There are 20 million of codelines in the Linux project for hardware drivers., so the total amount of different devices is 2k. It is equal to a large museum fully equipped with computer hardware from floor to ceilling. In addition, modern computer hardware is more complex than previuos models For example the avarage mouse is equipped with infrared sensors while the typical mouse in the 1990s was using a simple rubber ball to detect the movements. So we can estimate that in the future the complexity will grow further which results into more different devices which have a need for more codelines.