These things depend on location. Where are you? Here's how things work in SV.

Cover letter is exceptionally useful. It should explain in simple English why you are qualified for this particular job, highlighting relevant experience and showing that you actually bothered to figure out what this job/company is about. Doing this well is a huge advantage. At your level 1 out of 100 applicants will do it right. Cover letter should be just natural part of the body of the email and be in plain text. It should not be formal.

Suit with no tie is appropriate if you are interviewing for a job of CEO. Suit with a tie is never appropriate. This is different on east coast.