How 'wild Digimon' in the Digital World fuse or DNA digivolve? Digimon with human partner can easily fuse/DNA digivolve using digivice, like Omegamon in Digimon Adventure Movie.  

My question is how 'wild Digimon' fuse/DNA Digivolve? Let's use Royal Knight's Omegamon as an example. How did he came to be?
Digivolution is essentially a lot quicker when it forms a bond with a human. Like you said,  with a Digivice, a Human could share his or her energy and go about facilitating the same.In the case of wild Digimon, you could consider Digimon to more or less be computer data. Hence, they could constantly download additional data from the environment which would help them become stronger. If Digimon were to get deleted, they would essentialy break into many pieces of data which can be loaded by other Digimon. This potentially would trigger a Digivolution.