When a card enters the realm as a copy, e.g. or , its characteristics and abilities match the copied card. If the copying card (e.g. Selfsame Simulacrum) leaves the realm, it reverts to its printed characteristics and abilities.
When you create a copy, e.g. an artifact via , or a magic via or , the characteristics and abilities match the characteristics and abilities of the copied card, and the copy is a token. In the case of a copied magic (which is a token), it will be banished as soon as it resolves from the storyline, per the normal token rules.
By default, whoever creates the copy controls it and makes any necessary decisions for that copy, e.g. a player chooses to create a copy of so they control that copy of the spell.
A card's characteristics and abilities are its printed characteristics and abilities, by default. However, if a card "entered as a copy" then its characteristics and abilities are what's printed on the copied card. For example, if you play a and have it enter as a copy of a nearby , at the end of the turn it will summon a new Locust, not a new Simulacrum. Additionally, if you play a , copying a nearby , and your opponent then plays their own Mirror Realm copying your first one, the new Mirror Realm will be a Bedrock.