How to call a server function without tRPC?

Hi! I'm kinda noob on the T3 stack. I created a t3-app without nextAuth and tRPC. However, the scaffolded code doesn't cover a use of the server side code and I wasn't able to figure out how to do it. I tried to export a variable from the server directory, but I got the following error in the app page:

Error: ❌ Attempted to access a server-side environment variable on the client
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