An achievement is awarded each time you set a 1st, 2nd or 3rd personal best time on a segment. You also get an achievement for getting a KOM/QOM or top 10 overall placing.

A crown indicates a KOM/QOM for the fastest overall time on the segment.

A trophy indicates a top 10 overall placing

A medal indicates a new personal record, personal 2nd best time or personal 3rd best time

https://support.strava.com/hc/en-us/...t-s-a-segment-

If your brother only gets 1 achievement then it means he either doesn't have many segments where he rides or he isn't improving much as a rider. If you are getting 20 achievements for each ride then you might have a lot of segments where you live, keep riding new routes or you are getting faster each time you ride.

You will find that as you ride Strava longer the number of achievements you will get each ride will get less and less. For example, if you ride a particular route that has 20 segments then you will always get 20 achievements the first 3 times you ride that route. You will get PRs for each segment the first time you ride it, the second time you will either set PR's or 2nd bests and the third time you will set PR's, 2nd bests or 3rd bests. After the first three times if you want to keep getting achievements then you need to ride faster.

You will never get any achievements for manual entries because these are not counted for segments.