I don't know whether they retired the stock they bought back or put them back into the treasury, but in practice it's basically the same thing. Shares are fungible.
The issuance of new shares requieres new legal docs, and is constrained by the article of incorporation. A company will have to amend their articles of incorporation to issue shares beyond the allotted amount decided at the time the docs were created/filed.
This sentence is contradictory. Shares are either fungible (they are), or they're not, in which case they are 'non-fungible', like "non-fungible tokens."
In this case they are, because one share is like another. If you and I both have boeing shares, we can trade one for the other, and still feel like we got a fair trade. Additionally Boeing has the ability to mint new shares. And if they do that, they mint shares like yours and mine.