My macbook battery is fully charged on menu bar but on Coconut Battery says charging
Everything seems normal to me.
Batteries are much more complex than most people think. There are many factors such as heat/temperature, actual charge, power draw, lifecycle/age,... that contribute into &quot;inconsistent&quot; measurements.

Why design capacity is not consistent, sometimes 74%, 75%, 80%?

This is completely normal and every battery experiences this, especially as they get older. This again depends on many factors and the number will decrease over time/cycles.

I'm just wondering, will my MacBook overcharge or over discharge or this is normal behavior?

When at 100% your battery will stop charging. It will start charging again, usually a couple percent below 100% if continually connected to power. macOS tries to smoothen the curve, i.e. it makes you think its constantly at 100%, which it isn't. This behaviour is normal.
There are many excellent articles that can explain the physical details much better than I can, but there isn't really anything you should/shouldn't do with your MacBook in terms of battery, except trying to not (over)heat it. So just use your MacBook, but don't put it in bright sunlight for example.