The idea that "written Chinese" just happens to be identical to written Mandarin whereas written Cantonese is also the same and we actually translate our spoken words into a different written form is just historical revisionism and linguistic chauvinism.
The plain reality is that modern written Chinese is the way it is because the CCP standardized Mandarin as the national dialect.
Even without the CCP, Mandarin would have become the national dialect and dominated Chinese speech, etc. The CCP merely continued on the work because it was so useful and the easiest route to go as there had been so much work done and a system put in place by the previous regime.
EDIT: https://cpb-us-w2.wpmucdn.com/sites.gatech.edu/dist/d/987/fi...
^this link loaded better, its the same document, seems to be from Georgia Tech.
For non-Chinese readers, the document says that the dialect from Beiping (aka Beijing) should be adopted as the standard and then lists rules on pronunciation, how to pronunce using a Latin-based script, etc.
More specifically, 他不在這裏 is a phrase that I could imagine being used in official written text in past dynasties. But 佢唔喺呢度 does not.
In informal correspondence, imperial officials may have written things like this, because modern Standard Mandarin is based in part on 官话, the "court dialect" [1]. But any official text would almost certainly have been written in Classical Chinese.
[1] https://en.wikipedia.org/wiki/Mandarin_(late_imperial_lingua...
If you look it up character by character in a Mandarin dictionary, you can interpret it. And if you look up the Cantonese sentence character by character in a Cantonese dictionary, you can interpret it.
No, neither pronunciation nor meaning are fixed, and phonetic drift in general is faster than semantic drift, i.e. people from different parts of China are more likely to ascribe different pronunciations to the same character than different meanings, although the latter is not uncommon. Canjobear above gave two examples where the Mandarin meaning has changed relative to the Classical Chinese meaning.
When people want to write something for which they don't know a character (either because none has ever been defined or they just haven't learned it or the pronunciation differs based on context and they feel this should be reflected by using different characters) they can create a new one based on whatever makes sense to them. The character 佢 was chosen because the word sounds similar to that written 巨 in Cantonese. Nowadays so many more people are literate in Mandarin that they're likely to substitute a character whose pronunciation in Mandarin is similar to what they want to write.