Oh no, it's not feminine plural xD It's a special adjective category.
(ο/η) δημοφιλής (masc. and fem.), (το) δημοφιλές (neut.)
(οι) δημοφιλείς (masc. and fem.), (τα) δημοφιλή (neut.)
So, since το νησί is neuter singular, it's δημοφιλές.
There is nothing about this type in the tips and notes, but we plan on adding the info to the new tree, as well as make a seperate skill for adjective categories that are a bit more uncommon, like this one. ^.^
Does this group of adjectives only have certain words in it or does this form depend on the word it’s describing?
Any noun can be used with these adjectives. It's just a special category. There are other ones too, like υγιής (healthy), συνεπής (consistent), ακριβής (precise), διεθνής (international) etc. ^.^
το δημοφιλες ειναι επιθετο δηλαδη ο δημοφιλης , η δημοφιλης , το δημοφιλες and it is used in order to characterise the word '' νησι' .