I would like to know how are game quests created for real big games?

quest largely consist of some initialization code (where to go, what to do, what to kill, etc). and some runtime code (are they there yet? did they win the battle? if so, spawn treasure, etc). scripts are a common way to make this data driven, allowing non-coders to write quests. quests often have "stages" or different states (i'm not there yet, i'm there but haven't killed the badguy yet, and so on), with different runtime code executed at each stage of the quest.

Is there a logic/flow to it?

yes. its controlled by the runtime code/script for the quest.

how not to have multiple quests clash with one another?

you have to write them that way. which means you must anticipate beforehand ALL the possible ways they might interfere with each other. the same way you have to trap out ALL the ways the player could screw up the quest. i find a good way to do this is to at each step in the quest, ask yourself, ok, what could possibly go wrong? things like the player returning for their reward, but the quest giver has been killed. making quest givers un-killable is a common - but poor - workaround for that particular situation.

Is there a UI Flow or something that can explain how these quests are created?

looking at a script or the code for a quest will tell you how it behaves. quests can be created a variety of ways: hard coded, scripted using a standard language such as XML, scripted with a custom script language for the engine, or perhaps even created with some type of custom quest or content / level editor for the game engine.

And I would also like to know if there's an online editor or software where I can test my quests? So far I create quests using excel.

you really need a game to test quests. excel can be used to design quests. but code or script is pretty much required to implement quests there's a difference between designing and writing (IE implementing) quests. one is game design, the other is coding / scripting / level editing / content creation. confusion can occur due to the fact that both designing a quest and implementing a quest are sometimes referred to as "writing" a quest.

I have no clue about coding.

you'll need to at least learn some sort of scripting or how to use some sort of custom editor tool from some game engine to make quests. and of course you'll need the game to test them in. its easy to fail to anticipate everything when writing quests. so they very often behave incorrectly in some manner the first time you test them.

Is this the right place to post this question?

seems like it to me. the "writing" forum would be more for things like the backstory for a specific quest. and the "programming" forum would be for questions like how to implement a scripting system, how to implement a variable quest dialog system, etc.