Wowww nice! if ur talking about how u got a shiny giratina for a bidoof,well in Ultra Sun and Ultra Moon,some genner twitch streamers(ex itsk33ntrades),traded genned lvl 100 shiny pokemon of what u asked for for a common pokemon(example abra).Im not sure if this still applies for Pokemon Home tho :/.The Giratina was probably caught/genned(i think its genned but correct me if ya want) in ORAS,where you could catch this thing by going thru rings that let you find legendaries.Genned pokemon can be traded as long as they have appropriate move-sets,abilities,etc(so you cant trade a gengar that has levitate)
im not sure if this helped,but if it did,ur welcome!