Happy New Year Jupiter,
In informal speech, hundreds or thousands (or millions) can mean ‘a very large number’ or ‘very many’, but literally, it’s more a question of maths, I think: tens = from 20 to 90, hundreds = from 200 to 900, and so on. So, hundreds of thousands would be from 200.000 to 900.000.
By the way, I would put your question like this, as it is an indirect question:
‘I would like to know what differences there are between the following expressions :’ or ‘I would like to know what the differences between the following expressions are’. [size=100][If more than two are involved in a united situation, between is used: ‘Between the four of us, we raised a thousand dollars.’ If a comparison or an opposition is involved, between is used: ‘There was great rivalry between the three colleges. It was difficult to choose between them.’" (Parle-Craig, Ruth, and Vincent Hooper. Barron’s 1001 Pitfalls in English Grammar 70). These are exceptions to the rule: Using “between” for only two, “among” for more. Besides, the Oxford English Dictionary notes, “In all senses, between has, from its earliest appearance, been extended to more than two”.][/size]
The noun ‘help’ is normally used in the singular.
I hope to have been of service to you.
Any further opinion is always appreciated.