Upstairs in the Pokemon Center?
If you trade Pokemon between leafgreen and ruby ruby will have your leafgreen Pokemon and leafgreen will have the ruby Pokemon both in the party they will not be sent to the PC unless you personally walk to the PC and do so.
you cannot catch celebi in Pokemon leafgreen. you can only get it through cheat codes or trading.
You must deliver the Ruby and Sapphire item to Celio at the One Island pokemon center in Leafgreen first. Become part of the Hall of Fame in Sapphire next. Now you can trade anytime, go upstairs in a pokemon center and talk to the lady who leads you to the Trade Center (middle lady for sapphire, right lady in leafgreen). Link up and you can start trading.
Only by trading from LeafGreen and/or FireRed
Only by trading it from Leafgreen/Firered.
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
Machoke, Kadabra, Graveler, Haunter... Loads...
Teddiursa is not obtainable in Leafgreen and the only way to obtain is from trading Teddiursa from either Pokemon Emerald or Pokemon XD Gale of Darkness.
Yes at the end of cerulian cave
No but it is essential to trading with other games.
No, except by trading from Ruby/Sapphire/Emerald.
No, you need to TRADE from RSE.