$corner.r.b$ there is site General Symbols - Typst Documentation with all symbols and search bar (you can put in latex name)
The general answer to finding symbols is this:
In your case, since you know the latex name of the symbol, the Symbol documentation is easier and gives you corner.r.b, as MAX2 has said.
Hello!
I know the existence of that symbol, however it looks terrible for this purpose.
It doesn’t look like an infix operator.
How is this should look like?
Maybe $angle.right.rev$ ?
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hmm, that would have been valuable information in the original post 
The symbols page lists corner.r.b as the equivalent to \lrcorner. The math class is closing; the LaTeX symbol is binary? Maybe the \lrcorner note does not belong on that symbol?
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Yes, that looks great! It’s not the same as \lrcorner, but that style is also used for contraction in places.