Difference between quotation marks ('single' vs "double") in QGIS

Double quotes indicate that the string represents the name of an attribute while a single quote is a literal string.

So in your first case you get NULL because you don't have an attribute called something.

Here is the answer directly from QGIS help:

column name "column name" → Value of the field column name, take care to not be confused with simple quote, see below

'string' → a string value, take care to not be confused with double quote, see above

Double marks refer to columns in the attribute table, single marks to a string value. E.g. CASE WHEN "something" > 100 THEN 'a lot' ELSE 'not so much' checks the size of integer values in your column and adds a string to fields in the attribute table based on that size.