Data Ravelさんのイラストまとめ

@Fairi_digimon92 Yes, but in Crusader was shown that Andromon created Digimon like Millenniumon in a Abandoned Laboratory and ended up planning an upraising along with his "brotherhood".

At least, we know that Andromon created Millenniumon.

In that case, the very Digital World has a copy of a Digimon in its database, and if Digimon are able to complete certain requirement (Or use some special mechanism) they are capable of downloading the data that they need to evolve into other Digimon.

However the amount and type of Data that a Digimon absorbs can influence in the evolution that they will get, meaning that Digimon can evolve in various types of Digimon

According to Konaka, each Card has a unique Decrypt Key information that is used to pull data from the Digital World. This process is similar to today's digital money, so the data is disguised by a program, the card unlocks this and allow the Digimon to load that data.

In Digimon Tamers we had the "Card Slash" system that was used for both upgrades and evolution. In the beginning the most common was using cards from the "Digital Monster Card Game" to upgrade a Digimon with the effects of the cards, Digimon's equipements or skills.

Just like with Gennai, we do have the backstory behind Apocalymon and Mille has nothing to do with it, at most if we really consider Mille being the "root of all evil", but even this would be a huge speculation knowing that Apocaylmon's backstory wasn't common knowledge.

"Millenniumon is the mastermind behind all the villains of Digimon"

This one tends to change in intensity. I saw this as being as Mille simply being the creator of Apocalymon up to beings like D-Reaper and Lucemon being Mille's fragments.

The true nature of the Mugendramon and Chimairamon that fused in Mille is unknown, as much as somethings are implied. However not enough to say that Mugendramon was Ryo's partner and that Digimon turned into a Dark Master due to the sadness for Ryo never going to the DW.

Since the games where Ryo appeared were released mostly only in Japanese, most fans from this side of the world only about about him and the game's content with information from fan websites and sometimes lies were told that are still believed, so let's analyze some of them.

And then, Ryo Akiyama appeared in Digimon Tamers with Cyberdramon in episode 28: "Friend or Foe!? The Legendary Tamer Akiyama Ryo". Ryo's last game appearance still was in D-1 Tamers, and now he was in Tamers and already spent 10 months in the DW, what happened ?