Where did you see that you can upload custom plugins and themes on WordPress.COM? Can you please provide the link?
Please check out the pricing information again: https://wordpress.com/pricing/,
The business plan says CUSTOM domain names(like example.com) and UNLIMITED premium theme.
There's no mention of uploading custom themes or plugins in any of the plans.
Read this line given in this support document: https://en.support.wordpress.com/plugins/
Uploading custom plugins is not supported on our service.
WordPress.com’s goal is to keep things simple. We take care of the technical details, so you can focus on creating content. As such, we are not able to support uploading of custom WordPress themes on our service.
As stated here: https://en.support.wordpress.com/themes/adding-new-themes/
So... even if you upgrade to any of the paid plans you wont be able to upload custom themes or plugins on WordPress.COM.
The goal of WordPress.COM is to keep things simple. So that the technical stuffs are handled by WordPress and we focus on creating quality content.
If your desire to have greater customization, then you might consider switching to WordPress.ORG-as self hosted platfrom to create websites, blogs, online stores and more... It also gives you the ability to upload custom themes and plugins.
Find out the differences between WordPress.COM and WordPress.ORG here:
If you wish to move your blog from WordPress.COM to WordPress.ORG, follow the instructions given here:
You can ask for help while creating your site using WordPress.ORG on this support forum: https://wordpress.org/support/. Because, this forum only supports the sites hosted on WordPress.COM.