So I feel like the author is just assuming the audience knows the tropes of the genre and are skipping them, which is why some people are very confused and others understanding it perfectly.
So for the sake of the former:
Basic Premise-
Our MC, Ashleigh, got isekai'd into a fantasy world as the second young miss of a prominent royal family. She lived fairly normally as a pampered child until three years ago, in the midst of her studies at the Royal Academy, when she was suddenly called upon by the Imperial Family, informed that she was actually the Chosen One, and was destined to kill the Demon King. Realizing that she is indeed the MC of an isekai, she decided to do just that, gathering a ragtag group of goodhearted misfits and slaying the Demon King that resided in the far North. Congrats, bad guy killed, end of the world averted, time to go home.
The MC plans to return to her homeland and live a normal life, which entails finishing her education, something she was unable to do in her first life. She wavers on this but gets bullied by her sister to go through with it. So, Ashleigh becomes the first re-enroller in the Academy's history, gaining her a lot of attention. But attention is the exact opposite of what Ashleigh wants, she just wants to keep her head down and graduate quietly. Too bad she's caught the attention of the second prince who has rightly deduced that there's something important about her, and is more than happy to put her into dangerous situations to find out what- and pressure her into siding with him in the struggle for the throne.
World-
This is a pretty stock-standard fantasy world, we have so far seen humans and beastkin, and while there is a Demon King, his true form looked to be more like an elf.
The story takes place in The Empire, which takes up the western half of the continent. In the North is the frozen lands once ruled by the Demon King, that the Empire is soon planning to explore now that it's finally opened up again. The only beastkin we've met so far are those from the Feline Tribe, who live separate from the city, and who have a reputation for being thieves, though public perception seems to have changed since a member of their kind joined the Savior to fight the Demon King. There's also the Church and the Magic Tower who are the standard isekai Church and Magic Tower.
So far as we know, the imperial family is made up of the King, the Queen, the crown prince, and the second prince. The crown prince is the one with purple eyes, he's a jerk and the MC has disliked him since the word "go". He has the Queen's backing and they seem to be close. The second prince is the one with yellow eyes, something that's odd for the imperial family, meaning he's definitely a bastard child. He's been left out of all of this Demon King-Killing business and has no clue that Ashleigh is the Savior, the only reason he's interested in her is because he overheard his brother and the Queen whispering about her, combined that with her family's power, pieced together that she's somehow important, and is trying to get her on his side. He is also a jerk and a manipulator and has been repeatedly setting up situations to force her to join him.
The fact that Ashleigh is the Savior has been kept hidden from all but a select few people.
The Demon King isn't as big as a deal in the Empire as you may think, he lived in the far North, away from the rest of society, and seemingly didn't do much except exist. Apparently no one really cared about him until after his death. When suddenly, mysteriously, he's the talk of the town, along with the Savior who oddly, mysteriously, almost as if it's been set up cannot be identified. The Demon King also transformed after being stabbed, the visuals and his last words hinting that he may have been cursed or something. Ashleigh doesn't really care about that though, and has no motive or incentive to question it. Poor guy.
Family-
Ashleigh is the second daughter of the Lute family, a prominent royal family with strong ties to the Imperial family. She has two older siblings, the eldest, Phillip, and her big sister, Carnellian. Phillip, due to one reason or another, was basically totally overlooked for the position of heir, causing him to develop an inferiority complex and struggle to gain his parent's attention. He was murdered by Carnellian in front of Ashleigh's eyes in the dining room for no discernable reason. Carnellian is a cold-hearted bitch who Ashleigh hates, however she is the head of the family, so she can't really oppose her since, publicly, Ashleigh has no power.
Socially-wise, the entire family's an enigma, everyone knows they have a lot of power, but not really why or what their deal is. The second prince is planning to use Ashleigh as an in to the family to undercut Carnellian, who is allied with the crown prince.
Ashleigh's parents were living in a second home that Phillip favored for several years (they're implied to not be around a lot) and died in a mysterious carriage crash on the way back a year ago. Publicly, Ashleigh joined them on that vacation, which was where she was for those 3 years, and has only recently returned after mourning and dealing with family matters.
In conclusion, a lot of the plot is implied and banks on you paying attention, as this is definitely just build-up until the inevitable world-shaking truth gets brought to light. The Church is definitely sus. The Queen is definitely sus. The Demon King was probably just someone who got in the Royal Family's way, got cursed, then got killed for propaganda. The Royal Family is definitely going to prop up some false Savior. The parent's death was definitely not an accident and Phillip's death definitely has more going on than we know. Like I said, it's assuming you know the tropes of the genre, and goes 'yeah, you've seen this a million times before, here's the unique stuff'. It's actually kind of refreshing for someone like me who has read far more fantasy-isekai stories than feels appropriate to admit. But, if you're expecting a formal explanation, it makes sense like you'd feel like you're just left waiting.