No, a book or magazine in your local library is not public domain, and just
because it’s scanned online doesn’t mean it’s legal.
As I said, those magazines were published with the intent for people to copy the
plans and use them, and should include something in them saying that it’s legal
to do so (and may say something like “legal to use for personal use” which would
mean that selling things made from those plans would be questionable)
It’s only in the public domain if it was published before 1928 (or 1929, I don’t
remember exactly) or had copyright not renewed in a small window in the 50’s
(this is why the movie McClintok is broadcast as much as it is, the owners
failed to renew the copyright on it in this small window)