How do Wikipedians label articles by their quality level?
There is a template used in article talk pages: https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Marca_de_projeto
that allows defining the project topic, quality and importance. The importance level is project specific, but quality is overall. There is an automatic quality classification (https://pt.wikipedia.org/wiki/M%C3%B3dulo:Avalia%C3%A7%C3%A3o) that can be overuled by editors.
What levels are there and what processes do they follow when labeling articles for quality?
Article quality follows the general quality scale https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Escala_de_avalia%C3%A7%C3%A3o. It goes from 1 (poor quality), to 4 (ok quality). Quality 5 is reserved to "Artigos bons" (Good articles: https://pt.wikipedia.org/wiki/Wikip%C3%A9dia:Artigos_bons) and * (star) is reserved to "Artigos destacados" (Features articles: https://pt.wikipedia.org/wiki/Wikip%C3%A9dia:Artigos_destacados). The same scale applies to list articles. The top two classes are community based discussions, while the bottom four are individual decisions made by editors (or the automated classifier I mentioned before)
How do InfoBoxes work? Are they used like on English Wikipedia?
Infoboxes normally call the Info Module (https://pt.wikipedia.org/wiki/M%C3%B3dulo:Info) or use the Info template (https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Info).
There are some list using Wikidata properties (using primarily the https://pt.wikipedia.org/wiki/M%C3%B3dulo:WikidataIB)
Are there "citation needed" templates? How do they work?
Yes, you can mark a sentence as needing citations by using https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Carece_de_fontes, or a whole block (https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Carece_de_fontes/bloco). You can mark a section or whole article with https://pt.wikipedia.org/wiki/Predefini%C3%A7%C3%A3o:Carece_de_fontes. Several other types exist (see https://pt.wikipedia.org/wiki/Categoria:!Predefini%C3%A7%C3%B5es_sobre_fontes_em_falta)