The occasional swarm makes Magnemite available in front of Fuego Ironworks for Diamond. It can be regularly found there or other places in Platinum, or traded from Heartgold and Soulsilver.
In Pokemon Diamond and Pearl you need the National Pokedex to get Magnemite. In Pokemon Platinum you don't need the National Pokedex to get Magnemite.
The only way to get Magneton is to evolve a Magnemite.
You can catch it at the power plant
yes. migrate from a gba game. (magnemite is in all gba Pokemon games)
Swarm occasionally, in front of Fuego Ironworks.
They're very common at the Power Plant.
You cannot catch Celebi in Pokemon Diamond.
yes, you can catch Pokemon in Pokemon diamond
You can find a Magnemite in Pokémon Diamond at a Swarm in Fuego Ironworks with a Rarity of 40%.
neither of them is better, they are both the same, but in diamond, you catch dialga, and in pearl, you catch palkia, and you can catch different Pokemon in each game for example: you can catch this Pokemon in diamond, but not pearl but: you can catch this different Pokemon in pearl, but not diamond
You can catch makuhita in the fighting area in Pokemon diamond
I think you find it in fuego ironworks near floaroma town (you will need to have a Pokemon that knows surf) use poke-radar and do long chain until you find a magnemite.