My Electabuzz which i traded for from my friend gains boosted exp. I want to know how this works, why it is happening, and if i can do the same with my other Pokémon.
The Electabuzz you got from your friend is receiving boosted experience because it was traded. This is to encourage trading with others and over the Internet, though it comes with the caveat that if you don't have all 8 badges yet, the Pokémon may not obey your commands.
The exact boost in this case is probably
1.5x, since you described it as coming from a friend, so I assume you both live in and have your games purchased from the same real-world country. If you got a Pokémon traded from another country (i.e., over the GTS), it would instead have a
You can't get this same boost for Pokémon you've caught ("my other Pokémon"), but you can gain some other boosts in Generation IV:
1.5xfor being a Trainer battle (this is so common it's not marked as "boosted" by the in-game text)
1.5xif holding the extremely rare (in Generation IV) Lucky Egg