When should I be using jake brakes and the retarder?

Recently installed ATS. My friend who suggested it gave me basic advice on gears, jake brakes, the retarder, etc. I have little to no vehicle knowledge, so I figured I should ask what is ideal and where.

He told me I should be using the retarder in 'congested areas,' such as cities with lots of traffic lights (helps with deaccelerating and to avoid wearing out the air brakes), to passively assist with maintaining speed. Jake brakes, on the other hand, are just, like, general use brakes? I guess? It also helps with deacceleration (even more effective when combo-ed with the retarder from what I've experienced). However, I fail to see a significant difference between the two. Are they interchangeable? What are their practical uses?