If an item needs to be declared on customs for any country when arriving by airport, and you declare that item, does that necessarily mean that you will need to pay duty/taxes on that item?
No. Declaring an item simply means telling the customs authorities that you have it. Whether or not you have to pay duty will depend on the particular laws of that country and the situation.
For example, I purchased some cake at Demel in Vienna and brought it to the United States. I declared it to customs, because it is food, and the United States requires that you declare food. I didn't have to pay duty, because I did not exceed my allowance for goods that can be imported without duty.
Exactly what must be declared and when duty must be paid is a matter of national law.
For the UK, customs falls in the area of HMRC (Her Majesty's Revenue and Customs), and making a declaration does not always result in having to pay.
Your declaration might be irrelevant, i.e., something they are not interested in like a newspaper you purchased abroad. It might be relevant, but within the personal limits, like the limit on cigarettes. Or it might be relevant and over the limit, but it falls within their discretionary latitude. They will often use discretion if the variance is small or appears to be a one-off. So a declaration can have several outcomes that lead to not owing something.
There are also lots of times when the station at the "red line" is unmanned altogether. I have actually had to go looking for an agent to make a declaration and once even had to call them at another airport and ask them to send someone over (I was bringing back two bottles of Port which needed duty paid). But I suppose you could make a reasonable case that an unmanned station means the government accepts the forfeiture of any duty owed.
does that necessarily mean that you will need to pay duty/taxes on that item?
No, a definite conclusion cannot be made.
You are required to declare certain items so the Customs could track their temporary import status, to ensure compliance with import laws (such as agriculture), and to ensure you're not bringing restricted substances over the allowed limit. For example in certain countries you can bring only limited amount of alcohol/cigarettes (and extra won't be allowed even with duty). And in USA you are allowed to import counterfeit articles within limits, but you must declare them.
Then you're required to declare imports over certain value/combined value. In this case duty is calculated based on this value and you may need to pay it. This is generally at discretion of custom officers, as even if you exceed your duty free allowance but the amount is a few dollars, they still might let it through.
And if you declare imports over certain value as temporary imports (i.e. expensive laptop/phone as personal goods), the Customs need to ensure those are indeed temporary imports. There are various means to enforce this, but generally they won't apply to you unless you're bringing too many of those.