When a noun ends in -ou, I've read that it can be followed by both -s and -x, (des bisous, des bijoux) so, how can you know if it should be -s or -x ?
That's a very interesting question. Most nouns will take an -s in the plural form. However there are exceptions are you've mentioned.
Take a look at these 2 links:
Most nouns take an -s in the plural unless they already end in -s, -x, -z (the second video in particular - 7 exceptions)
I hope this is useful.
Bonne journée !
I think the following link may help you:-
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