You get an egg by grinding the digimon u want an egg from though it may take alot of time because the drop rate is severely low the first time i played i think i had to grind for an hour to get a kunemon so if ur gonna grind for an egg make ABSOLUTELY sure you want to put in the work or u can buy one from a player that is selling them which usually costs alot of money so ya theres ur answer
There's also two different eggs. And among those there are a multitude of sub eggs. There are digieggs which, after scanning, give you data. Those can be aquired from any digimon. Different digimon drop different egg types so make sure you know what kind you want. And as the one above answered, you can get mercenary digieggs from grinding the digimon itself or buying them from other players.