How can i add js file in sharepoint modern site.

You can create Application customizer(Extension) to load your external JS files.

You can load SPComponentloader to load your script.

Create Extension: https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/get-started/build-a-hello-world-extension

 

load external JS example:

https://sharepoint.stackexchange.com/questions/213236/spx-and-typescript-how-to-add-references-to-sp-js