SQL server data format mismatch question

New to SQL server, curious about the mismatched data type situation. Instead of spending a couple of hours to test a variety of situations with the possibility of missing some scenarios, I want to tap into the experience of the experts for a quick learn.

In some programming languages, when two different formats meets, for example,  a 2 byte number type meets a 8 byte number type, then all results will be the 8 byte, automatically. We are safely assume and call the resulting variable using the 8 byte number type. Same for some text format as well.

So in SQL server, what is the rule for auto convert to bigger data type?