You have a fixed width on quite a few elements:
To fix this edit the CSS to something like this:
As for where to make this change I can't say without seeing the theme, it will likely be a SCSS or LESS file.
Now your site is responsive again:
In theme module-overrides you can omit the view/frontend folders in the path, making the path to your overwrite file as follows:
You can not characteristics like this.
The best idea to use Magento luma theme or build a simple theme base on luma.
Every theme has a lot of extensions, which also need to remove.
Best to use luma or build a simple theme base on luma. And customization on this new theme or luma theme.