how can Linux be a moderated product to sell for desktop

It kinda depends on each individuals' use case; there's lots of different Linux distributions that are better (or worse) for specific workloads.

Any given laptop I'm staring at in a store will probably work perfectly fine as a general-use machine with Linux Mint installed. This is my go-to distro when repurposing a machine because it works great out of the box. If I were running a computer store and wanted to sell consumer laptops with Linux on them, I'd default to Mint.

If someone is looking to turn their PC into something more specialized for gaming, they can look at something like Bazzite or Batocera. These will generally require some tinkering.

If an individual or company is looking to build an office with many workstations and user accounts, they might consider Red Hat Enterprise Linux so they can benefit from official support channels if something needs troubleshooting. Many computer labs at NCSU used RHEL when I attended many years ago.

Want a stable server environment? Debian is a standard pick.

Want a barebones system with no bells and whistles (but great battery life)? Alpine oughta work.

So Linux has many options for end users to pick from, which can be seen as a good thing (more options is generally good), but also a bad thing (many end users might consider the plethora of options to be overwhelming if they've never used Linux before).

Linux (or is called unix?)

Linux (Or GNU/Linux) operating systems are a modern implementation of an old research OS that was called "Unix". Spiritual successors to Unix like Linux and BSD try to bring a lot of the design philosophies of Unix into modern OSes (I believe this is generally called the "POSIX" standard. e.g.: macOS is a POSIX compliant OS, iirc).

If I've gotten any of this information incorrect, please don't tell Richard Stallman.

It's a tough sell because there is no monetary incentive to get Linux on laptops and desktops. Dell has a few computers that ship with Ubuntu, and Lenovo with Fedora, and there's System76. The problem is that the big manufacturers (namely Dell) get push-back from Microsoft if they start to sell other OSes with their products, so they no longer have 100% domination. Microsoft will say "Oh you're selling a few products that come with Linux? Well, we won't offer you the ability to sell Windows anymore..." which would obviously be a huge impact to their business. They have gotten around this, but their offerings are still really slim. The market just isn't there compared to Windows based computers. Shelf space is expensive so they go with what sells: Windows based products.

Some laptop manufacturers (and at least one of the larger ones) already offer Linux (Ubuntu) as a pre-installed OS. I suspect this will become more common if/when Linux becomes more popular as a mainstream desktop OS. Most likely it will still be 1 or 2 pre-selected distros though even then.

One way to do it is for each company to develop their own flavor to ship with their laptop, in much the same way phone manufacturers just modify Android and ship it.

As an example, check out System76 and their laptops featuring their Pop!_OS distro, which is very user friendly and stable in my experience.