API endpoint to show/add/modify/delete content I would like to access and modify certain entries using asynchronous calls from a custom made webapp using React which is complementary to the main website. Is there any way to achieve this in Craft CMS?
I've got 0 experience with React, but you can write a Craft plugin (generate boilerplate code at https://pluginfactory.io), that has a controller action(s) your webapp can post to.

From there you can use EntriesService-&gt;saveEntry (craft()-&gt;entries-&gt;saveEntry()) to update/create entries just like Craft/any other plugin does.