Mastemon

Q:What order do a [Security] effect, "when [...] performs a security check" effect, and "when a card is removed from [...] security stack" effect activate when they trigger simultaneously upon a security check?

A:[Security] effects take precedence for activation. Upon a security check, a [Security] effect will immediately activate without pending activation. For other triggered effects, the turn player activates their effects first.

Q:Can I use this card's [All Turns] effect to place either my Digimon or my opponent's Digimon as the bottom security card?

A:Yes, you can.

Q:Does this card's also trigger when this card's [All Turns] effect is used to place this card itself as the bottom security card?

A:No, it doesn't trigger.