I never know what people mean by absolute truth, it sounds like
they're referring to a truth above all truths, but there are many
What about the scenes you 'see' while dreaming? If I could ask you without waking you, whether all the scenes are true/truth? You would certainly say, "Yes". Sometimes the person who sleeps with you would have experienced the gravity of the truths in the dream (Just for an example...not about you). But if I asked you about the scenes in the dream, what would be your response? You would smile. There is no relation between what you see now and what you saw in the dream. Similarly, because of misconceptions we don't know the absolute truth. But there were/are people who realized this Truth. And even the sinner has the right to realize this Truth at this moment (and it must be so).
What about the many truths you experienced in your dream? Is the absolute truth similar to other truths? "No". But you could say one thing--"The first person, second person and the third person or in other words, everything in the dream were none other than you. When you take the word 'truths' in the plural form even all the truths are nothing but the things in a dream, isn't it? (or you realized all the truths were only the 'truths in a dream') This is happening in advaita. (This is not about Monism. I believe this might be the idea you are seeking by the term 'Monism'.)
So I was wondering if there were a lot of philosophy that associates
the absolute truth to the idea that everything is one thing and the
truth of that one thing is the absolute truth.
The idea--"everything is one thing" would take you to monism because when you say 'one thing' you do not consider you as the part of that one thing. The term 'advaita' solves even this trifle-but-significant problem.