When two or more players in a room, each two of them are connected via either Relay or P2P (peer to peer) method.
A player can see which method is used to set up the connection between them and another player in the room, by looking at the player name list. If it says "Relay" then the connection is set up through Relay. Otherwise it's P2P.
Relay method means one of the relay nodes placed around the world will be used to relay the network packets between them.
P2P method means they will be sending network packets to each other directly. Since there is no dedicated 3rd node for the packets to go to, P2P method usually incurs less delay/lag, so it's preferred. (Its effect on connection jitteriness is not clear though.)
By default, the game will try to use P2P to set up the connection.
However, in some cases, especially when both players are behind routers, the game cannot set up P2P connection and will have to revert to relay connection. When this happens, the players can configure their router to open up the port. In essence, packets sent to the router's external UDP 13497 port needs to be forwarded to the UDP 13497 port of the VR device's IP. Usually this router config option is called "port forwarding" or "port mapping".
For Quest (or other devices that uses "randomized MAC address" by default), it's recommended to also (in Quest) go into the detailed wifi hotspot view, and change "use randomized MAC address" to "use device MAC address". Otherwise the Quest device's IP address can change and render the port forwarding rule configured in the router ineffective.
On a related note, if a player sees the ball bounces on the opponent side of the table multiple times and then the opponent still manages to return it, it's because the two players are likely over long distance, so it takes longer time for the packets to travel between them than it takes for the ball to bounce twice on the table. In this case, it's strongly recommended for the player to turn on Pingball feature which will somehow sync the ball activity with the opponent's activity.