Learn English – How do “you” pronounce eczema

My online dictionary shows that aargh is pronounced with the "r" sound, and either with a hard "g" or a silent "g" at the end.

I imagine that's the point of the "h" - to allow the "g" to go silent.

aargh |är; ärg| (exclamation) used as an expression of anguish, horror, rage, or other strong emotion, often with humorous intent.

First, British English speakers do not all pronounce things the same way. There is a lot of dialectical diversity in Britain. As you saw in the comments, nickson104 is a Welshman and he pronounces "want" a different way from what I describe in my answer. No one pronunciation is absolutely "correct," since you'd need to specify in what context you mean it. However, I guess you mean to ask about the usual pronunciations for foreign learners in particular. That makes things easier, since resources for teaching British English as a second language tend to converge on a standardized or generic variety. Throughout this answer, I'll refer to "standard" AmE and BrE, by which I basically mean the forms that dictionary pronunciation guides and the like tend to record (and that foreign students tend to learn in classes).

How "want" is pronounced (according to a British dictionary)

The Oxford Learner's dictionaries list the British pronunciation of "want" as /wɒnt/.

Context on what these symbols mean

The word "can't" is not pronounced with /ɒ/ in any dialect of English, as far as I know. It seems you mean [ɑ]: this is the vowel sound generally heard in "father," "car" (if you pronounce it without an "r" sound), and the American-accent pronunciation of "not."

In standard American English, the vowel in words like "lot, not, stop, robot" (we name the vowel in this class of words "LOT" for short) is pronounced the same as the first vowel of the word "father" (named FATHER); the symbol for the American pronunciation of both LOT and FATHER is [ɑ]. For this reason, many pronunciation guides based on American English don't use the symbol /ɒ/ at all, and always use /ɑ/ instead.

However, in standard British English, the vowel used in FATHER words (like "father") is different from the vowel used in LOT words (like "not, lot, stop").

The FATHER sound, /ɑː/, is pronounced pretty much as in the American pronunciation of the word "father." The vowel is "long," literally: it is generally held for a longer period of time than the LOT sound (that is what the symbol /ː/ means after a vowel).

The LOT sound, /ɒ/, is pronounced somewhat like a short version of the vowel in American "father," [ɑ], but more like a short version of the vowel sound in American "law," /ɔ/; if "law" has the same vowel as "father" for you, use a short version of the vowel sound in "fort" instead, but remove the "r" from the end of it.

(These are just approximate descriptions; the important thing is to be aware that there is a difference so that you can listen for it and learn more about it if you need to.)

The word "can't" has the long [ɑː] sound of "father"; it does not have the short [ɒ] sound of "lot". The word "want" has the short [ɒ] sound of "lot"; it does not have the long [ɑː] sound of "father".

General advice for studying pronunciation

Google doesn't pronounce words very naturally and sometimes has mistakes. A better way to learn pronunciation is to study English phonology so that you know what sounds and distinctions to listen for (especially the distinctions that aren't in your native language), then listen to native speakers in Youtube videos and things like that, and if you want to know how to pronounce a specific word, look up how it is transcribed in a dictionary that gives pronunciation and listen to it on a site like Forvo.