Do we say the 90’s burgers, the 90s’ burgers , or do we simply not add an apostrophe to it?
You are, I think, confused by an evolving orthographic convention.
Fifty years ago, when I was in school, it was an almost universal convention that the plurals of numerals and individual letters (and often, but not consistently, initialisms) acting as nouns should be represented with apostrophe+s, ’s:
the protest movement of the 1960’s … of the 60’s (or, sometimes the ’60’s!)
people in their 40’s
mind your p’s and q’s
Since then, however, style guides and publishers have gradually embraced representing these plurals with bare s, just like ordinary nouns:
the protest movement of the 1960s … of the 60s
people in their 40s
mind your ps and qs
But there are still many writers and publishers who were brought up under the old convention and cling to it, so you still find both styles.
I recommend you adopt the new convention, which appears to be dominant now and avoids confusion with possessive ’s. But you might also consider spelling out numbers which can be represented as single words, and capitalizing individual letters:
The protest movement of the 60s … of the sixties
people in their 40s … in their forties
mind your ps and qs … Ps and Qs
The quarter-pounder was a 90s burger … a nineties burger