The designation of friend or foe is only relevant to the spell that asks you to choose. A player you call your friend doesn't become your teammate, and the next “friend or foe” spell you cast could name that player your foe.

You make this choice for yourself as well as each other player. In some rare cases, you may wish to call yourself (or your teammate in a Two-Headed Giant game) a foe. You can do that.

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

If the copied creature has {X} in its mana cost, X is considered to be 0.

If the copied creature is copying something else (for example, if the copied creature is an Evil Twin), then the token enters the battlefield as whatever that creature copied.

Each token copies exactly what is printed on the original creature and nothing else (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

Friends perform their specified actions before foes. This means that a friend's action may cause a triggered ability of a foe's permanent to trigger before that foe's action causes that permanent to leave the battlefield.

You may call a player a friend or a foe even if that player will be instructed to perform an impossible action. For example, for Regna's Sanction, a foe may control no untapped creatures.

When Zndrsplt's Judgment resolves and you choose friend or foe, first the next friend in turn order (or, if it's a friend's turn, that friend) chooses a creature they control, then each other friend in turn order does the same, then each of those players create a token at the same time. Friends will know choices made by earlier players when making their choices. Repeat this process for foes, who choose a creature one at a time and then return them all to their owners' hands at the same time.

If the copied creature is a token, the token that's created copies the original characteristics of that token as stated by the effect that created the token.

Players can't take actions in between the time that friends perform their actions and the time that foes perform theirs. If any abilities trigger from friends performing actions, those triggers won't be put onto the stack until the foes perform their actions and the spell finishes resolving.