WP.com hosting of WP.org developed blog

an existing blog and domain…WP.com…
Can I keep WP.com as my host…but still use WP.org to edit its appearance, add plug-ins, etc.?

No.

are there instructions for how to download an existing site and sending to another host?

https://www.google.com/search?q=how+to+move+from+wordpress.com+to+self+hosted

Or, WordPress.com will do that for you for a fee…and that might be your best bet since the Dashboard exporter/importer can be quite quirky at times and seldom (if ever) brings uploads along.

I have accomplished downloading and installing the WP.org software…

Just curious: A local installation on your own computer for development?

Can I keep WP.com as my host (instead of hosting my own) but still use WP.org to edit its appearance, add plug-ins, etc.?

No, WordPress.com is not a hosting provider, so that wouldn’t be possible.

If I cannot, are there instructions for how to download an existing site and sending to another host?

Sure, just follow this guide: https://en.support.wordpress.com/moving-to-a-self-hosted-wordpress-site/

As for the domain, you can either keep it with WordPress.com and point it to your hosting provider or transfer it: https://en.support.wordpress.com/domains/move-domain/

Thanks to both of you! @leejosepho: Yes. Local for personal development. It seems overtime I want to do something different with my blog, the only way to accomplish it is to use a plug-in/alter the PHP code. I think I can get some things by learning how to alter some of the CSS code but somethings (like adding a “follow me with Bloglovin” button) seem to only be possible with a plug-in.

Everything you can download from www.ads-software.com is free, and there is nothing wrong with editing a plugin or two into your own as long as you still give credit where it is due.

You’re welcome!