Figma is a great tool which happens to have a great community behind it. And that's important because whenever there are things which are not doable by default in the editor there's always plugins made by users - most of the time entirely free to use which can help. 

How do I get symbols in Figma?

If you're looking to get keyboard symbols such as the copyright © or approximately ≈ or any other symbols - you can just search the symbol on google and just copy paste it inside a text box in Figma.

The only requirement is that the font you use has that specific symbol designed. Most of the google fonts have them but there are a lot of custom fonts which only support the default letters and numbers so your symbols might get replaced by placeholders.  

To be sure your google font has a specific symbol you can go to the google font website and paste your symbol in the "type something" box along with your selected font. If your symbol shows up it means it's supported. 

On the other hand if you're looking to add icons in Figma you can use plugins with open source icon packs. I've made a post about it a few weeks ago:

Where is the icon library in Figma?

It will teach you how to run plugins like Iconify and find symbols and icons for pretty much anything you need. There's thousands to chose from.

To summarize - if you want to get symbols added in Figma all you have to do is copy them from google and paste them in a text box. The only requirement is that the font you're using actually has your symbol drawn. If you were looking for icons instead, you can use plugins such as Iconify to find and import SVG icons directly in the editor and customize it however you want.