If two or more players are involved in maintaining a loop across turns, each player chooses a number of iterations to perform, or announces their intent to continue indefinitely. If all players choose to continue indefinitely, the game is a draw. Otherwise, the game advances through the lowest number of iterations chosen and the player who chose that number receives priority at the point they stop taking an action to sustain the loop.

Is a player who is doing nothing on their turn other than drawing and discarding, with the discard happening to be a Nexus of Fate (/Progenitus /Darksteel Colossus /Emrakul with no other cards in graveyard) against a player with actions other than game mandated ones, Such as Teferi Hero of Dominaria (/ Academy Ruins) also “involved in maintaining a loop across turns?”

How do the loop rules handle the aspects that the hand's contents are always hidden despite the meta knowledge that there must be at least one and no more than fore Nexuses of Fate?

The Teferi player eventually (i.e., before Slow Play applies) has to choose to lose this game.

This loop includes the Teferi player taking an action; they can choose to take a different action. The Nexus player is simply doing what the game rules instruct them to do - yes, they're choosing which card to discard, but it's not a game action they're taking, it's following the rules of the game.

It's an odd outcome, and if someone rules this as a draw, there's no one in Policyland that's going to yell at them, or even say they're “wrong” - but if it comes up, and you're paying attention now, you'll know what to do.

d:^D