Indefinite pronouns

We use indefinite pronouns to refer to people, places or things without saying exactly who, where or what they are. We use pronouns ending in -body or -one for people, ending in where for places and pronouns ending in -thing for things.

https://learnenglish.britishcouncil.org/english-grammar/indefinite-pronouns

https://www.ef.com.es/recursos-aprender-ingles/gramatica-inglesa/pronombres-indefinidos/

https://learnenglish.britishcouncil.org/english-grammar/indefinite-pronouns

https://www.curso-ingles.com/aprender/cursos/nivel-intermedio/relative-and-indefinite-pronouns/indefinite-pronouns

And some exercices to practice

https://elt.oup.com/student/solutions/int/grammar/grammar_08_022e?cc=global&selLanguage=en
https://agendaweb.org/grammar/anybody-someone-indefinite-pronouns-exercises.html

https://www.curso-ingles.com/practicar/ejercicios/indefinite-pronouns