How do Wikipedians label articles by their quality level? What levels are there and what processes do they follow when labeling articles for quality?
There is a grading scheme, which comprises of such levels: IV → III → II → I → Good article → Featured article/list. Afaik, all wikiprojects use this same scheme.
The process for an article to achieve Good or Featured status is the same for any topic. An article has to be nominated and voted for.
All other articles have a chance to be graded only if i) they are in scope of a wikiproject, ii) the wikiproject is alive and works on grading. 124К articles have a template, corresponding to en:Template:WPBannerMeta, transcluded on the talk page; not all of them are graded.
Template:Stub and its derivatives, used in the main namespace, roughly mean "this article is not complete"; they can be placed not only in level IV article but also level III and even II if an article is generally lacking information.
How do Infoboxes work? Are they used like on English Wikipedia?
Editors are encouraged to use infoboxes. The most used ones tend to take a lot of info from Wikidata (e.g. many biographies, especially translated from other languages, use Infobox person with no parameters at all). Infoboxes that are used less often or are specific to narrow topics with data not present in Wikidata, are more likely to be filled in when used in an article.
Are there "citation needed" templates? How do they work?
Template:Citation needed would be most used among inline dispute templates and Template:Unreferenced and Template:Refimprove -- most used among non-inline templates.