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Your original question was

Is "Some African countries" or "African countries" the antecedent of the relatives clauses in both 1 and 2?

1. Some African countries which are very poor have to be helped by international organisations.
2. Some African countries, which are very poor, have to be helped by international organisations.

Let me first touch upon a "non-identifying (= non-defining; non-restrictive) clause". These clauses do not identify or classify; they simply tell us more about the antecedent (person, thing etc) that is already identified. The use of comma before and after the non-identifying clause helps in its clear and unambiguous identification. In simple language, try to read the sentence without the clause between the two commas. If the sentence is meaningful and grammatically correct, it confirms that the clause between the two commas is a "non-identifying (= non-defining; non-restrictive) clause". This is the case in your sentence 2. Sentence 1 is incorrect as the "non-identifying" clause does not have a comma before and after it. It is emphasized that an "identifying clause" is not separated by commas. This is because the noun would be incomplete without the identifying relative clause.

Note the expression "that is already identified" in the preceding paragraph. The antecedent is "Some African countries". "Some" is an indefinite determiner. "African" is used as an adjective of the noun "countries". So the antecedent is "countries". "African" pertains to the countries being referred to. The phrase "Some African countries" functions as an antecedent of the "non-identifying (= non-defining; non-restrictive) clause". I wouldn't agree with the implied inference that 'all African countries" are poor". Drag0nspeaker has already mentioned that there are some African countries which are not "very poor".

I hope it helps.

OOooh! Awkward questions . .

1. In the first one there are three different ways of 'organising' into phrases, I think. With no commas, two of them don't work really
Therefore, I would say that "African countries" is the noun-phrase modified by "which are very poor" - then the whole phrase "African countries which are very poor" is quantified by "some".

There are fifty or so African countries. We define the narrower group as "African countries which are very poor", then narrow it further to just some of those.

2. In the second, I see two possibilities which work. Both of your suggestions.

a. We take some of the 54 African countries. We note that these countries are poor.
This makes "Some African countries" the antecedent of the relative clause.

b. We note that all 54 African countries are poor - then choose some of them.
This makes "African countries" the antecedent.

Grammatically, it could be either.
Truthfully, there are some African countries which are not "very poor". Equatorial Guinea (the 'richest' by GDP per capita) is almost on a par with Spain, Italy, New Zealand and the EU as a 'nation' - and well ahead of many other European countries.

So logically, it must be (a) - the antecedent is "Some African Countries".

I would guess that the police went out to arrest specifically "people who had committed offences".
They don't go round randomly arresting teenagers (not often anyway!)

It's not an irrelevant parenthetic bit of data.
It doesn't really make sense like this:
The police arrested three youngsters (who, by an odd coincidence, had committed crimes).

I'd say it was a restrictive clause.

My (non-native's) understanding is that which / whichever applies to situations where you choose among discrete options. You don't have to necessarily know all the options (dresses in your example) in advance.

There are many planets in the Universe. Whichever one you choose, be careful to manage it well.

But:
Whatever you manage, manage it well.
Here "it" refers to just anything - from planets to romantic relationships, so it's not a discrete array of options.

There is a finite choice of dresses, and you choose one

I could choose whatever looked nice on me (anything)

I could choose whatever I wanted

I could choose whichever dress looked nice on me. (that one)

There is a finite choice of universities, so 'whichever' sounds right to me. 'Whatever' and 'what' are commonly used where some people would use 'which' though.