feed manager has a setting for excerpts

blog also has a setting so that the feed can be set to only have an excerpt

so you can configure an excerpt in either one of those places, but you have more control if you do it in the blog and manually create the excerpt for each post.

No, you don't need to do it programmatically nor should you code against the blog or any other feature.

I told you look in the settings for the blog and in the settings for the feed manager. If you look you will see the settings, then try them.

You can enable excerpts in the blog and you can enable excerpts within the feed manager so you have 2 ways of doing it.

If you look at demo.mojoportal.com right now, I have create 2 blogs and on the home page I've created 2 feed managers.

The first feed manager is using Excerpt mode in the feed manager and consuming the feed from blog 1

The  second feed manager is not using Excerpt mode but it is consuming the feed from blog 2 and blog 2 is configured for excerpt mode.

When the blog is configured for excerpt mode then you will see a new tab in the blog editor page where you can edit the excerpt.

The blog exposes a feed link (it only works after there is at least one post in the blog), right click the feed link and choose copy link location. Then paste that as the feed url in feed manager, put the url for the blog page as the web site url and whatever you like for the title.

If you click the edit links for the feed manager on the demo site you will see what I entered there.

The feed link in the blog is right below the calendar

right click it and get the feed url

I just told you where it is. You only need to add it once to the feed manager not for each post, I already added it so you don't need to add it on the demo site you just need to understand what I have already told you.