Rulings
Q1: What does a card with "X in its text" refer to?
It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, Link, or Assembly requirements. For example, a card with Knightmon in its text would include cards with the name DarkKnightmon and cards with the text Knightmon in their effects.
Q2: Can I use this card's On Play When Digivolving effect to give "Start of Your Main Phase This Digimon attacks" to a Digimon that isn't affected by effects?
You can give that effect to such Digimon, but if the Digimon isn't affected by effects upon the trigger timing, the gained effect won't trigger.
Q3: Can I place 1 of the specified cards from my trash as this Digimon's bottom digivolution card for this card's On Play When Digivolving effect?
No, you can't. A "by" condition can't be met if only some of the required actions are performed. In order for the conditions of this card's On Play When Digivolving effect to be met, you must place 2 of the specified cards from your trash as this Digimon's bottom digivolution cards.
Data Sources
Lv.6 Arcturusmon P-240 P mega | Virus | Dragonkin/VB <Collision> (During this Digimon's attack, all of your opponent's Digimon gain <Blocker>, and the opponent blocks if able.) <Piercing> (When this Digimon attacks and deletes an opponent's Digimon, it performs any security checks it normally would.) <Reboot> <Blocker> [On Play] [When Digivolving] <De-Digivolve 3> 1 of your opponent's Digimon. Then, by placing 2 cards with [Gammamon] in its text or the [VB] trait from your trash as this Digimon's bottom digivolution cards, give 1 of your opponent's Digimon "[Start of Your Main Phase] This Digimon attacks." until their turn ends. [On Deletion] You may play 1 [Proximamon] from your hand or trash without paying the cost. --- [Opponent's Turn] [Once Per Turn] When one of your opponent's Digimon attacks, you may change the attack target to this Digimon.