Learn German: Ich lese das Profil der Freiwilligen. - I am reading the profile of the volunteer f.

Freiwilligen is a nominalized adjective, meaning it is an adjective acting as a noun. Because it comes from the adjective freiwillig (voluntary), its endings follow standard adjective declension rules instead of normal noun rules. In this sentence, it is in the genitive case after the definite article der, so it requires the weak adjective ending -en.

Why do we use der if the volunteer is female?

In German, the feminine definite article die changes to der in the genitive case (which often translates to 'of the' or shows possession). So der Freiwilligen tells us that the profile belongs to the female volunteer.

Could der Freiwilligen also mean 'of the volunteers' in the plural?

Yes, it absolutely could! The definite article for plural nouns in the genitive case is also der, and nominalized adjectives in the plural after der also take an -en ending. Without extra context, this sentence can translate to either 'of the volunteer (f)' or 'of the volunteers'.

How would this sentence change if the volunteer were male?