Aberrant Mind Sorcerer

An effect that says "choose a target, then roll a d20" or similar still uses the normal process of putting an ability on the stack and resolving it. Choosing targets is part of putting the ability on the stack and rolling the d20 happens later, as the ability resolves.

Dice used must have equally likely outcomes and the roll must be fair. Although physical dice are recommended, digital substitutes are allowed, provided they have the same number of equally likely outcomes as specified in the original roll instruction.

An ability that tells you to roll a die will also specify what to do with the result of that roll. Most often, this is in the form of a "results table" in the card text.