How to help new employees build a sense of belonging in a short period of time?

It takes about half a year for a newcomer to get rid of the mentality of an outsider, and it takes three years or even longer for a person to truly understand his or her own company. But the most difficult problem to solve is actually the understanding and integration of a newcomer to the company culture. How to make new employees integrate into the existing company at the fastest speed? How to achieve the optimal result in the input-output ratio of newcomers?