What is that everyone like in Frieren? I think its boring.

I have yet to find an instance in the show that actually uses the concept of going on a journey again as an undying elf to reconnect with a late friend in an engaging way. Anytime it happens, as with the ring

i don't see instances like the ring (among many others, i'm just building off your example) as meant to reinvigorate Frieren's connection directly to her late companions, at least, not in the way that i think you're trying to articulate. it's not her past relationships that act as the crux of the narrative, it's the way that they mold her ongoing relationships with Fern, Stark, Sein, and the rest of the ancillary cast that enters and exits as the show progresses. Frieren begins to witness (though maybe not entirely understand, as of Episode 14) how that one-hundredth of her life plays a role in how she deepens her connection to her newfound companions in ways like giving Fern a present, or being persistent in her convincing of Sein to join her adventures.

any flashbacks to the past amongst these catalysts of present change reference events that the viewer is already implicitly privy to — such as the events which transpired when Himmel and co. tried to recruit Frieren, it was already established that she was both a part of the party to begin with, and that she doesn't fully cognize the significance of human existence to the sheer abundance of time she has. the viewer knows both of these elements already, and the show then uses such to mold the ongoing events taking place in the present time, with my example paralleling Sein's episode. in other words, it's not Frieren's reminiscing and recognition of the past that's the main goal of the story — it's how it shapes her present.

most things are just explained to you, a good example of this is the episode where Frieren kills Aura. An entire episode dedicated of flashbacks where Flamme meets Frieren and explains to us in excruciating detail

a fair point. i don't particularly view exposition like this as destructive to the worldbuilding, because the other elements of the world i find are fulfilling enough to compensate for the infodumps with respect to mechanics and systems — though i do understand that it can be a pain point for some. that being said, exposition is unavoidable in any work, and amidst the context of high-octane battles, i do think it's much more agreeable in these situations as opposed to the more slow-going episodes, where there isn't as much of that. this is where i have to disagree with your subsequent points:

The show has two layers of narrative, one in the present and one in the past, yet there is no way they can be distinguished without the context of which figures aren't present.

it's a bit disingenuous to truncate the progression of the world merely through the dichotomy of Himmel and co. or no Himmel and co., demon king or no demon king. there are many examples of how the "before and after" of the conflict shaped the ways various settlements and their villagers organize amongst the present. Frieren meeting an old man that upskirted her 80 years prior, or confronting changing standards amongst the magic system and the various tokens used to accredit prowess over time are natural and more nuanced consequences of how change in the show is portrayed via everyday circumstance. of course the emergent backdrop of less danger via demons is a key hallmark of the past versus the present, but much of the more quotidian happenstances demonstrate that this is far from the only benchmark of progression.

The world it presents is flat and uninteresting, as everything shown in it only serves and revolves around the plot. Any detail that could be revealed in a piece of dialogue is omitted because the writer didn't bother to think of something to put in there and rather went with the most generic line manageable

i'm curious to probe your mind as to how you understand the "plot" of the show to be. of course, the present-day Frieren adventurers are on a quest to reach Aureole and find Himmel and Flamme, but i find that this is merely the premise that kickstarts the current adventure and gives the show a linearity to work with. the plot is something far less quantifiable in a tidy sentence; what good does it do for Frieren to reach Aureole if, in the present, she feels the exact same way about her relationships as in the past? she'll have made the arduous journey to have no character development or cognizance about the weight behind human life and its legacy beyond death. the reunion would be thematically useless.

these aforementioned circumstances of change and Frieren's evolution of her conception of time and life via her past experiences is in essence the plot — what the show tries to advance — so i don't see any of its explorations of that plot as flat or uninteresting, because it dives into the more microcosmic reflections of how the plot is shaped in the current world that the cast finds itself in. demons aren't as big of a problem, so diplomacy has evolved. humans have more time to experiment with magic, so they've learned to refine skills like flight and have integrated spells like Zoltraak into a standard repertoire of any given mage. these events i find lend substance to advancing the plot, so simply because they all converge on developing such isn't a knock against it if it's done effectively, which i feel it is. of course, you're well within your rights to not find any fulfillment within this show whatsoever, but i do believe it holds more far more merit than you identify as being present.

Oh, and the show is almost unbearable to watch for anyone speaking German

everything in this show has already been done better 5½ years ago when Kyoto Animation aired their magnum opus, Violet Evergarden

i can't engage with either of these to a meaningful extent, with the former being because i don't speak German; and the latter because it's been more than two years since i saw Violet Evergarden, so i don't remember enough about it to fairly evaluate it in comparison to Frieren. sorry about that.