In terms of historical origins for these carriers, there are some possible explanations for the Tier IV-VIII ships.
The Tier IV (Rhein) could be a speculative variation on the proposed Jade-Class carrier conversions (Also named after rivers), which were to be built from the liners SS Potsdam and SS Gneisenau. In terms of complement, this design would make sense for a Tier IV, and the single image of Rhein looks to have a hull reasonably mercantile in origin. However, it would make for the largest Tier IV carrier (Based upon the specifications for the Jade-Class) at some 17,000 long tons displacement, and the biplanes would make no sense for a WW2 design. It could also be the liner Ausonia, which the Kaiserliche Marine had proposals to complete as a carrier in 1918. However, the carrier design seen in the images looks far more modern than these proposals. Despite this, using a heavily modernized variation of the Ausonia would somewhat justify the biplane complement.
The Tier VI (Weser) is the most extensively historical ship in this line (Partially completed during WW2). It started as the Hipper-class CA Seydlitz, but had conversion work done starting 1942 and was renamed to be Weser. It was never completed and was scuttled before the end of the war. The depiction of this ship within the images we can see matches previous renderings of the Weser as completed, including another forum post on it found here. This carrier would have a complement of 10 BF 109T and 10 Ju87E Stuka (Both navalised variants), and fits well into Tier 6 in terms of size. However, it seems to have much less plane capacity than the other Tier 6 carriers.
The Tier VIII (August Parseval) has a hull that looks suspiciously similar to that of the Graf Zeppelin, so it could very well be an extensively modified design of the planned second GZ-class ship with an altered secondary armament, or the canceled 3rd and 4th. Given its extensive similarity to the Graf Zeppelin (The bow is almost identical), it's extremely unlikely that this is the Europa liner conversion that was proposed during WW2.
There is no explanation for the Tier X, and it is probably a completely fictional battleship conversion or new build.