I'm working on a portfolio site as a favor. I'm trying to dig into the template in order to modify what's displayed in a portfolio category page but I'm just stuck. The site is using Twenty Seventeen with Spectra & Visual Portfolio plugins installed. Basically what I'm trying to do is have only excerpts on the category page and the full portfolio item on its dedicated page.
This is the site in its current state:
http://www.kristinamico.com/portfolio-category/feature-writing/
If anyone can point me in the right direction I'll be extremely grateful. At this point I'm not even clear about which file is building the above view.

You will need to create template in the theme. I recommend using a child theme in order to not get overwritten by updates to Twenty Seventeen in the future.
You can either create your own archive.php file or if you need different archive pages to look different, create one for the portfolio_category taxonomy. See template hierarchy for details: https://developer.wordpress.org/themes/basics/template-hierarchy/
After that (I recommend copying the archive.php from the theme to get you started with your custom archive template), you need to create your own logic here https://github.com/WordPress/WordPress/blob/master/wp-content/themes/twentyseventeen/archive.php#L43. Probably something like this to only show the excerpt: https://github.com/WordPress/WordPress/blob/master/wp-content/themes/twentyseventeen/template-parts/post/content-excerpt.php
there must be an option to show only the excerpts.