Sorry if this a dupe, but this always confuses me. Who would win if a Pokemon were to use an explosion move and it takes out all Pokemon on the battlefield? Would you or your opponent win? Does it matter who uses the explosion move that determines victories?
The player that uses Explosion will lose. This makes sense because your Pokémon faints first from the move.
Source: https://replay.pokemonshowdown.com/gen8doublesou-1332976815 (If Slower)
(If faster) https://replay.pokemonshowdown.com/gen8doublesou-1333625988
If Ally is slowest in field: https://replay.pokemonshowdown.com/gen8doublesou-1333631825
Using Explosion in that sort of scenario will cause the opponent to win. The Pokemon that uses Explosion faints first, then your ally faints next, followed by both opponents. Initially I thought it would be decided by speed, but no.
First replay (The ally Diglett is faster than both opponents)
Second replay (The ally Happiny is slower than both opponents)