Do we use fusion in real life? why or why not?

Yes..? Kind of.

Explanation:

Fusion is the combination of lighter nuclei into heavier nuclei. Yes, this is seen in nature; the sun. The sun is an example of fusion. However, on earth, no. To fuse 2 nuclei, you need extreme temperature and extreme pressure to fuse them together because the nuclei are the same charges. So, yes, but not really?

when it is at an intermediate population density