Hi,
1. Did you talk the “Filter” option? The filter is for page portfolio template not in BM Pagebuilder or Gallery list layout
Page > Add new, select the one of the 3 template:

The filter option will be shown:

2. In single “gallery format post”, you can put the intro text in WordPress default editor:

3. Could you please paste a sample page URL for “text and images are side by side”?
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Ps: Maybe you don’t understand the different of Page and Post.
The each single project is built by Post(Posts > add new), set as Gallery format, use gallery list layout to create. refer “Post” of DOC: https://seatheme.net/documentation/arnold/index.html#!/posts
The portfolio grid(list) page is built by Post(Pages > add new), choose page template to create, refer “Page”: https://seatheme.net/documentation/arnold/index.html#!/pages