Feel free to copy/paste. Or, if you're using a computer, you should be able to see the little emoji button on the toolbar.
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If you are using a mobile, you probably have to click the three dots on the toolbar to give you the additional options.
"Classical building" is just over halfway down under the Travel section.
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Once you use it a couple times, it should add it to the top of the emoji menu under "recently used". Or, like I said, feel free to copy/paste!
edit: Oh snap! It's HUGE if you copy/paste! I'm not gonna miss that!!!
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edit2: Oh yea, just remembered, you can also type ":classical(underline)building:" and it will pop up with the option.