Is my cassette made of steel?

Focusing on the question, almost all cassette cogs are steel. For Shimano’s and Campagnolo’s top road groups, the top few cogs are made of titanium to save weight. However, titanium wears a lot faster than steel, thus this is limited to Dura Ace and Super Record. Those cassettes are very expensive. Your bike seems to come with most of a Shimano 105 group, which is two levels down from Dura Ace. That cassette is fully steel, aside from the aluminum cog carrier for the largest 3 cogs and the plastic (I believe) spacers. (NB: your brakes are Tektro and the cranks are FSA, and down-speccing these items is a common cost-saving measure. I wouldn't be that surprised if a manufacturer down-specced the cassette, but there's no Shimano 11s cassette below 105 in the current generation of parts.)

For MTB cassettes, Shimano’s XTR cassette has the top 3 cogs made of aluminum and the next few made of titanium. XT has its largest 2 cogs made of aluminum. The balance are all steel SRAM's X01 and XX1 (highest end two groups) may have only the largest cog made of aluminum, with steel for the rest. However, those cogs have 39 or more teeth. So, they’re larger than many road chainrings, which are all made of aluminum (although MTB chainrings, being smaller, are often made of steel, and sometimes titanium rings may be available aftermarket). Generally, the more teeth there are in a cog or chainring, the more material there is to spread wear over, so it's feasible to make big cogs or rings of aluminum or titanium and not have them wear too fast. I'm not sure how fast big aluminum cogs (or mid sized titanium ones like on XTR) wear on MTBs. Otherwise, on road groups, you’ll generally only see aluminum cogs on weight weenie builds, and these would not be regarded as daily use cassettes.

However, you indicated that the instructions for the chain checker said to use the .75 wear indicator if you have aluminum cogs. Maybe something is getting lost in translation, but I don’t think that can be correct. Chain manufacturer guidance is to change the chain at 0.5% wear if it’s 11s or higher, otherwise change at 0.75%. It has nothing to do with the cog material. You would change cogs if the chain were skipping, and if you change the chain right when it hits the indicated amount of wear, you can run multiple chains before you need to change the cassette. I am not sure why an aluminum or titanium cassette would necessarily influence the point at which you should replace the chain. It does seem like the steel chain would wear at the same rate or at a lower rate if used on a softer material, and both titanium and aluminum are softer. As to the 1% wear mark, that may have been a thing on 8s or earlier chains. Once a chain exceeds its wear mark, the roller-roller distance will be significantly different from what the cogs and chainrings were designed to accommodate. The chain will then begin to wear out the cogs and chainrings much faster. If you left your 11s chain to 1% wear, you would certainly have to replace the cassette at the same time, and you could have worn the chainrings significantly as well.

Regarding the German Wikipedia page you linked to, I don't speak German, but Google Translate does render Kettenverschleißlehre as "chain wear gauge." The page does appear to say that, using Google Translate:

Most manufacturers recommend changing the chain if the elongation of the chain exceeds 0.75 or 1%, depending on the type of chain. Accordingly, chain measuring gauges are used to check whether the elongation is within the tolerance range 0.75% (for light metal cassettes) or 1% (for steel cassettes) elongation.

However, that statement is unsourced. As I stated, I don't believe it is correct. The author might have mistakenly confused things with chain wear guidance from the 8s/9s era, but again, they would be incorrect as to modern chains.

You didn't ask, but I believe that some chains are partly made out of titanium. For example, YBN has one chain model with titanium rollers, but I believe the pins and plates are steel. This is a super-duper weight weenie item. I would expect it to have extraordinarily short life - chains 'stretch' because the rollers and pins wear out from moving against each other during pedaling, and titanium is softer, so it should wear faster in this application, just like ti cassette cogs. However, again, you would still want to replace the chain at the 0.5 wear mark. Also, there are some carbon fiber chainrings (e.g. Fibre Lyte) available for weight weenie builds, although I also don't believe they would wear very well.

The shape of the tool is very suspicious, as are the wear limits.

According to http://pardo.net/bike/pic/fail-004/000.html, most chain wear tools measure the wrong thing. They measure both pin wear and roller clearance although only pin wear affects cassette and chainring wear, the only reason why you'd like to replace your chain.

Because of measuring roller clearance too, an inaccurate tool can report a new chain to be worn, and can report an overly worn chain to be still within acceptable limits.

A proper tool should have two measuring points and one tensioning point. The measuring points should be at the same side of rollers.

Your tool seems to have three points but the shape at the left end of the tool, presumably what should (could?) be the tensioning point, looks very suspicious. Nothing at all like Shimano's TL-CN41 which belongs to one of the two only series of tools I know to be accurate the other being Park Tool's only good indicator, CC-4 -- other Park Tools wear indicators are junk. TL-CN40 and TL-CN42 are two other tools from the Shimano series, TL-CN42 has slightly different tensioning mechanism that is still accurate.

So there's a risk that you get inaccurate measurement using your tool. I'd recommend getting Shimano's tool instead.

Also the wear limit should be 0.5% for steel cassettes. At 0.75% you probably will be spending more money on cassettes and chains than you would be at 0.5% because your cassette lasts much less but your chains slightly more. The wear limit of 1.0% could even wear your chainrings, way too high.

For aluminum cassettes, the wear limit actually doesn't matter. It'll be worn anyway no matter what you set the wear limit to, if the whole cassette including the small sprockets like 11T are aluminum. I think the ultimate limit for aluminum cassettes is to what elongation you dare to wear your chain considering the chainring wear.