True Polymorph causes the first target permanent to copy the printed values of the second target permanent, including any copy and mutate effects that have been applied to it. It won't copy counters on that permanent or effects that have changed its power, toughness, types, color, and so on.

If either target is an illegal target for True Polymorph as it resolves, the spell will do nothing.

If the first target becomes a creature the same turn it entered the battlefield, you can't attack with it or use any of its abilities unless it has haste.

If the target permanent is an Equipment attached to a creature and it becomes a copy of a non-Equipment permanent, it becomes unattached. If it becomes a copy of an Equipment, it remains attached.

True Polymorph's effect does not have a duration. The copy effect it creates lasts as long as the first target remains on the battlefield.

If an effect begins to apply to the first target permanent before it becomes a copy of another permanent, that effect will continue to apply.

If the first target permanent copies a permanent that's copying something else, it will become whatever the second target is copying.