What are the purpose of medals? and what happen? if I will get the platinum medal.

Answer 1

You receive xp for achieving badges, that's all that happens really. You will get platinum if you gained 20k xp.
It is not useful for purposes outside SoloLearn.
The xp system exists to keep you motivated to learn, post code here, contribute and take challenges.

Answer 2

Same purpose achievements have in any video game. Just shows overall progress and maybe helps motivate you to do more.

If you get Plat, you get plat. Everyone gets to see what badge you are (gold, silver etc..). So, while the higher the badge doesn't mean much, you can surely assume someone whos plat has been using Sololearn for quite some time. They may also have a fair amount of programming experience. That's about it tbh.