In video 1, regarding combos, it is system based and before you start the gameplay, the system will determine the total power, character stats, character equipments, skill bonuses, skill effects and AOE percentages and attacks as well as other factors that can be viewed in heroes page. The combo meter is filled up with damage over time accumulated, whether the character attacks or took damage, it fills up the combo meter, and there is an option there in later stages where you will let the system automatically activates their respective ultimate skills or manually.
Every character of the game information can be viewed in heroes tab and you can check them One by one in portraits view, as well as their general stats when in Ascended tier and in a sample max level. (Tiers like Ascended can be higher than that displayed, reaching to 5 star ascended, making it more OP), and character levels reaching level 280 or maybe more.