How are people encountering legendaries?

Actually in Unite, you cannot "share" a Pokemon like you would in many other games. It's set up so that you earn a "license" to play with the specific Pokemon.

"License Points" are necessary to have to unlock the license to use a specific Pokemon. After accumulating enough License Points to unlock the License along the License Ladder (path) then you can play with that Pokemon in all the stadiums except for special stadiums such as the Johto or Kantos stadium (where only specific Pokemon are allowed to play) or when someone else chooses them before you or it's banned (in certain matches) in the pregame lobby.

License point requirements to unlock the ability to use a specific pokemon vary.

As for the legendary birds and others .. like Articuno, Moltres, Zapdos, they were offered for a limited time through special events for free IF you earned them by completing missions. I obtained them all without using money.

If you missed the events, you can still earn them via the License Ladder Path thingy, as a special selection (when you get to pick), or when you finally get high enough to unlock them, or you can pay for the license (I think) with gems right away. Gems cost real life money.

I'm sorry, but Unite does not allow trading licenses with other trainers. However you can gift up to one License Point per day for five trainers on your friends list. Those points do not come from your License Points previously accumulated.

Hope that helps clarify things!