Can I do a android game without downloading UNITY Android optional addons?
nope. otherwise what would the point be? You might be able to make a web game, and access it with an android browser, haven’t tested this though.
There is no optional addon to download.
Its part of the Unity installation itself.
But you must buy an Addon license to enable and use it
as for the other input: There are no plugins on mobile browsers, as such a webplayer will NOT run on the mobile no matter what you do