What does the word "most" mean?

I own 3 houses. I spend 40% of my time in house A, 35% in house B, and 25% in house C.

Can I say that I spend most of my time in house A? Or do I have to spend more than 50% of my time in house A to be able to say that?

If the former is true, it means that I spend most of the time in house A, but at the same time I spend most of the time (60%) outside house A. How can this be?

If the latter is true, how do I say in simple words that I spend the largest part of my time (which is still < 50%) in house A?

Or another example: China is the most populous country in the world: about 19% of all people live there. Can I say that most people live in China?

Edit: This question is not the same as this one (Is “most” equivalent to “a majority of”?). In my question I ask how to refer to the largest part of something when it's smaller than 50%. It's been established that the word "most" does not apply here because it generally means "more than half".

Edit 2: Elon Musk knows the word 'plurality', but the other guy doesn't: Video

Most people don't live in China, but more people live in China than in any other nation.

You can say that China is the most populous nation.

You do not spend most of your time living in house A, but you spend more time living there than anywhere else.

If you add the definite article it becomes correct. You spend the most time living in house A.

It's a subtle difference and the technical explanation will have to be supplied by someone else.

Edit: Thinking about this a bit- in the first case you are comparing the case of the time spent in house A with all of your time, and in the second case you are comparing the time spent in house A with each time spent in a similar classification of time segment, so time in house A vs. time in house B and so on.

These collection words are not technical. They don't specify an exact percent. They are vague and fluid.

One can count items and determine percentages exactly, but most (all? see below!) collection words work with amounts that are inexact.

But they do have some relative informal strengths The following is an ordering of some collection words in English with an explanation for each. Assume that '...of the time' follows, e.g. 'I spend none of the time at house A'

• none - No time is spent. You are perceived to be wrong or lying if you have spent any time at the house at all.
• little - hardly any time at all. This is sort of relative. Presumably something else in the discussion has more.
• a little - towards the smaller amount. slightly different from simply 'little'. It is not as relative as 'little'. Parallel to 'few' and 'a few'.
• some - An unspecified amount, neither implying a lot nor a little. This is the vaguest one of all. All that is implied is that it is not all and it is not none (because pragmatically, you would have use one of those extremes if it were the case). An academic might say "I have some publications in that area" to be a tiny bit misleading because they only have 2. There is a technical meaning of 'some' which is 'not none' or 'at least one'. So logically one can have one out of a million and still have 'some'. But the natural inclination is that it is more than one.
• a lot - this is like some neither none nor all, but also not few. After that it can be anywhere from more than a few to almost all. There is no comparison here or rather it is a comparison of feeling rather than number. That is, you can truthfully say 'There are a lot of tall people in this room' if everybody is of one height and a handful of tall people stick out.
• much - I find this synonymous with 'a lot' but is a little more formal register (but still vague and non-technical).
• most - this is in the direction of 'all' and is comparative, meaning more than any other. The general idea is that it is 'almost all' but because it is so vague, it can be used for anything that is more than anything else. If there are two things to compare, then this must be more than 50%.
• all - absolutely every bit and nothing else.

(for simplicity's sake, I've left out 'several' 'few' and other similar terms for 'count nouns')

Except for the two extremes, none and all, they are all vague to some extent. There is no specific number or ratio that must be adhered to. They vary depending on context. The ordering given shows their expected order in general but one may be more than the other in one context and maybe the other way in another context.

Also, these words show the difficulty in dictionary definitions. One online definition for 'most' is "greatest in amount or degree." with a single sample "they've had the most success". But this is actually in the context of 'the most' not the simple 'most'. The definition is close but doesn't specify all the subtlety.

The related technical words are majority and plurality. 'Majority' means '>=50%', as in elections. 'Plurality' means a higher percentage than any other choice, could be but not necessarily a majority.

Your instance

I spend most of my time at A

is not wrong. If someone did not know the percentages, they'd probably assume more than 1/2 your time was spent at A, but if they saw a tabulation, they would not think you are wrong.

It looks then like 'most' is equivalent to 'plurality', and I think they are very close. But 'most' is still vague. Like 'a couple' (which literally means two), 'most' might be used aspirationally and is used often just to mean 'a lot'.