The indigenous outposts are supposed to be military settlements of native peoples.
I think someone said though that they are only supposed to be buildable in the east as they represent the Ptolemaioi and Seleukids recruiting natives when no Hellenic settlers are available and are used for other factions such as Hayasdan and Saba to eventually convert to eastern imperial from tribal culture.
As for the other military settlers building line, those are Hellenic settlers and will boost Hellenic culture, but they require colony points which you can currently only get from controlling a metropolis.
e: Also if you don't currently have colony points buildings which require such will not show up in the building browser.
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