>>58667553It does vary by series, but generally speaking, the two models are this
A: The alternative universe model
The digital world already existed in some form as a parallel universe. When humans developed the internet, it wasn't actually a series of tubes, it was boring a hole into another universe. This was first contact for both, digimon feed off of data the same way humans breathe and eat, and the tremendous quantity of data humans pipe through the digital world quickly became an incredibly significant aspect of life to Digimon.
Or
B: The human genesis model
When humans made the internet, programs began assimilating data in unexpected ways, gained sapience, and began evolving on their own. They created their own world, and exist there.
Some stories are a bit of column A, bit of column B, there's an oddball series or two that does something completely different, but most can be boiled down to one of these summaries.
In either event, the data from humans, amd the human world is an incredibly valuable resource to digimon, which in turn makes humans a subject of great interest to digimon. For digimon, data exchange and assimilation is everything. It's food, it's air, it's war, it's sex. When digimon enter the human world, or humans enter the digital world, things begin breaking down a bit in terms of everything becoming a digital signal. A digimon eating real food is breaking it down into data, a human eating digital food in the digital world is assembling that data into nutrition, there's a degree of interchangeability between these two alien extremes of reality.
Human emotions are some of the most complex and dense data around, though, this is why digimon that become close with humans can rapidly evolve to levels that otherwise maybe .001% of digimon reach. But, because that data is emotion, and digimon assimilate data, they also become influenced by those emotions, and that begins to influence digimon as a whole.