jQuery is not defined in SPFx extension

I have created SPFx extension (not a webpart, in case it makes any difference) and trying to add jQuery but its not doing it. I have installed it into package with

npm install --save @types/jquery

added

"externals": {
"jquery": "./src/extensions/tracker2/js/JQuery_3.5.1.js"
},

into config.json ,

import * as jQuery from 'jquery';

into MyPackage.ts
But still getting " jQuery is not defined " when trying to use jQuery in my code.

Thanks