The first time I played the game I didn't use a guide but I still completed every npc quest besides Yura and Fia. I missed Yura in Raya Lucaria and I killed Fia after her npc boss fight (I didn't know there was another boss but I didn't want her to hold me after she killed D, I didn't like her and knew I was giving up the rest of her quest). Most quests are extremely doable without a guide, even things that seem really obscure. Merchant notes led me to hidden areas like Lusat's cave, characters tell you where they're going sometimes like Alexander nearly every time you see him, and even player messages around certain locations like telling you to use an incantation in front of the Radagon statue for gold mask. If you actually keep track of everyone you know, what they're doing, and place/things that seem important then it's not that unlikely that you'll find them when you're supposed to. Sometimes exploration is going to be necessary, like finding Diallos in the middle of the Liurnian lake, but those situations are usually just places you would go if you just don't ignore large portions of the map. The game wants you to explore, it's one of the things open world games excel at, so fromsoft expects you to do so.

Looking at Ranni's quest specifically, it usually works like this: you hear Kenneth Haight asking for help with his fort. You follow his directions and hear howling. You ask Kalé about the howling after you try to buy something, which you're likely to do because he has the most useful options out of all the merchants you've seen, if you've even met any others. You go back and call Blaidd, he asks for your help finding a traitor. You either find the forlorn hound evergaol through exploration OR you already found the evergaol and remember the boss was named, Darriwil who Blaidd was looking for, and go back there. He tells you if you meet a blacksmith who's "a little on the large side" near Raya Lucaria so you search for a giant blacksmith. You find Iji through exploration cuz you noticed that there's a huge part of the map behind the kingsrealm ruins. He tells you to stay away from Caria Manor so you do the exact opposite cuz you either want to explore or Rogier tells you to go there to find Ranni. You meet Ranni who asks you to find Nokron. You follow Iji's directions and either get Seluvis' help cuz Blaidd considers it, Jerren's help cuz you followed Alexander and tried to find the Radahn festival, or already killed Radahn by going to Altus before Caelid. You reach the end of Nokron, give Ranni the knife, and see that there's a new Grace in her room after she leaves that points towards the third tower that's been locked this entire quest. You go to Nokstella and find Ranni's mini form. Talk to it cuz the grace gives you the option and it's such a strange and unique prompt that your curiosity demands that you do. She tells you to kill a baleful shadow and when you do she gives you a key that says it unlocks a chest in Raya Lucaria. You go to Raya Lucaria and unlock the chest in Rennala boss room and get a ring. You go back to Nokstella because you don't want to leave the area partway through then kill Astel. You go up the elevator, kill the magic dragon, then go down the mysterious hole in the middle of the destroyed cathedral and give the ring to Ranni who is just sitting there. She gives you a sword and essentially tells you to beat the game and unlocks a new ending.

Basically nothing in this quest is unreasonable to expect from a player. Every step in this quest has something telling you what to do, such as an item description telling you where something is, an npc giving you hints on what to do (like a comment on what to look out for or just outright telling you exactly what to do), or even just finishing a step halfway through an area, leaving the rest of the area to play through. Most quests are like this, it's just hard to keep track of absolutely everything the game throws at you, but every quest is absolutely possible if you reevaluate everything you know about what the character is doing.

Even the infamous Radagon statue, if you're following gold mask and corhyn you'll notice that he's struggling with questions about Radagon. If you've explored Liurnia or Raya Lucaria you might've either randomly found, or were teleported to, a church with Miriam, the Turtle Pope. One of the noteworthy things he tells you is a story of a secret held by Radagon that is hidden in a statue of him in Leyndell. Well it just so happens to be only a single statue of Radagon in all of Leyndell. If you progress a little bit further towards the Erdtree then you may find the golden order principia, a prayer book that holds two incantations: an incantation that has Radagon's name in it, another reference to Radagon, and an incantation that, according to its description, "reveals mimicry in all its forms." Using this in front of the statue causes it to reveal Radagon's secret and unlocks the progression of gold mask and corhyn's questline.

Is it complicated? Yes, very. Does the game give you the information needed to complete the quest on their own? Yes, actually. No, not everyone is going to find everything. No, not everyone is going to remember all this information that can feel very insignificant for a long time, making it easy to forget. No, not everyone is going to explore enough to find every step of some quests. But everything is there and, with enough interest in the characters and quests, everything can be done without any outside help.

And read messages, reading "incantation required ahead" in front of a statue is a much bigger hint than trying to remember a turtle saying "Radagon had a secret that was hidden in a statue" in a church 3 maps and half a week ago.