They've been digitalizing media and only streaming it when it's outside of copyright. This means many 2050+ for recent music but ~1970s music and earlier is fair game.

Copyright for sound recordings is pretty complicated in the US. Recordings from before 1972 were protected by a variety of state level laws, so even recordings that should be in the public domain, like ones from before 1924, weren't guaranteed to be so until the 2018 Music Modernization Act[1], and even so have a 3 year grace period from when that was enacted. It gets even more confusing for recordings from 1924-1978, when rights holders had to file for copyright and then file for extensions, AND the law has changed numerous times for different dates within that range.[2] So no, it's not really fair to say that 1970's and earlier music is fair game.

[1] https://www.eff.org/deeplinks/2018/09/new-music-modernizatio...

[2] http://www.publicdomainsherpa.com/public-domain-sound-record...

Seems like he just has a large collection of records, not the original ownership rights to the recordings - so, no.

Then what will they do with the digitized content after it's been digitized?

I think the IA is considered a library by California so they could do it within the limits of what a traditional library loans media contents out.

They keep a lot of stuff that is currently unavailable to the public. Maybe one day it will be out of copyright and then they can publish it quickly.

In the mean time at least it is not lost.