The main author is always the first author but definetely other authors do also receive the credit in form of citations after the article gets published. But it usually depends on your requirement, for example if the institution has a requirement to have the student or supervisor to be the first author, then the order of authors should be made accordingly. Definetley if the student is first author then he must have enough contribution to the article.
Credit will go to all authors. However, the first author is usually seen as the lead for the study. The first author is normally the corresponding author.
corresponding author, most cases in journals corresponding author is considered as the first author other co authors irrespective of the position in publication
To me the first author is the main author of a journal and he/she will receive the major credit. However, if the corresponding author is different than first author then the corresponding author will receive the second credit and then all other authors will receive the credit equally. But this rule might be different in different context. Nowadays, many journals require to mentions the contribution of authors.