Can One Person Code His Own Word Processor with Capabilities Comparable to MS Word?

There are real issues of scale at play in this. To take an analogy: an experienced, diligent, and capable construction worker-type person could build an entire simple residential home or a cabin, with modern features like running water and electricity, and do it all by himself, from scratch. However, that person would be daunted if the home were a large mansion with a pool and a cabana / pool house on the grounds.

AND FURTHERMORE:

No one in his right mind would expect to start and complete the building of an entire big-city skyscraper of say 80 stories, all by himself.

With artifacts made from solid materials, this sort of scale issue is somewhat self-evident. It may surprise the software-naive person when I say that in software, analogous issues of scale apply.

Microsoft Word has probably (just a guess) somewhere in the range off 3 - 6 million lines of good working code (perhaps more if you count things that are part of the product, but in fact also are used and were developed separately for other products— such as Visual Basic for Applications for example). No one is going to write and debug that much code in one lifetime.

HOWEVER:

One can indeed write something “all by oneself” that delivers some (more or less large) part of what might be considered MS Word’s core functionality. One can program a macro-capable, fairly full-featured text editor “all by oneself”. In the past, I have advised intermediate programmers to write their own text editors, as an advanced-intermediate coding project. There is even a book "Software Tools in Pascal" that details the construction of a simple but quite capable, line-oriented text editor.

Then after that, one can program a markup utility, that will format text in

ways that allow an ordinary editor (creating marked-up text) to produce (for

example) a beautifully typeset book-length manuscript, using arbitrary type-

faces (type fonts and sizes) and page sizes and so forth. Here is a Wikipedia article

about existing markup languages:

https://en.wikipedia.org/wiki/List_of_markup_languages

Essentially all of those computer science types who scorn MS Word and insist on using

their own tools have used this approach. Perhaps the most sophisticated of the markup languages is Donald Knuth’s TeX (Tau Epsilon Chi, pronounced “tek”) and its companion LaTeX. There are dozens to hundreds of example “markup languages” out there for you to study—including capable ones that are open source. I would be tempted (by contrast with programming “my own editor”) to choose an open source markup tool, and then simply modify the open code to add a feature or two if it came up short in some respect, rather than coding one from scratch.

OTOH, doing your own (simple) markup language from scratch all by yourself is feasible, though you might find (as even Donald Knuth did) that writing low-level drivers to assure fully typeset, “beautiful text” from particular printers and output devices is is not worth your time, except perhaps to do one for one particular output device— mostly just to say that you have done so, and you can do more if you have to.

ALSO BTW, Dr. Knuth, who is certainly high among the most intellectually capable programmers alive in the world today, took most of a decade to complete Tex. Doing a fully capable “Microsoft Word-like program” might be feasible for you but it will (probably) not be at all quick and easy to get some result that you are happy with.