TLDR: What I want is best explained by the pictures below.
In words: I have partially overlapping features and I want to visualize this overlap with transparency. I know I can set a Layer Properties > Display > Transparent value, but this will only make the whole layer transparent, not the actual features.
Obviously I could make a layer for each feature and use Layer transparency, but that is not an option for >22'000 features.
I achieved the latter picture by using Layer Properties > Symbology > Advanced > Transparency (see below), selecting (somewhat randomly) the ID as the required "transparency attribute/field". This makes some features transparent, others not... probably a high ID gets a high transparency? I haven't figured out the logic, I also tried adding a new attribute/field "transparent" filled only with a value 33, to use this as "transparency field" (=33% transparency), however this doesn't work at all (features within one layer are still "not transparent amongst themselves"). I also added in one row with 0 and one row 100, leaving all others at 33, hoping that transparency is scaled to the value range. Both didn't work. Anyway, all this feels like a hack. I was hoping I could make the feature color transparent (e.g. as in QGIS), but can't see an option to do so. Using a transparent texture (e.g. lines) also doesn't work, since the lines are perfectly aligned, not showing overlap either. Is there a proper way to do this?