When I look to edit some of the fields and layouts I have in content blocks, I see placeholders such as: [[+main_left]] [[+main_right]] [[+main]] [[+value]]
I was wondering where I could find these placeholders to see more about how they work
For layouts, the placeholders are dependent on your Columns configuration. Edit the layout and open the Columns tab - the column keys there match your template.
For fields it can be a bit more complex as it depends on the input type. Most input types use a [[+value]] placeholder for the primary content, but more complex ones may have multiples.
Hovering over the template field (or its label) in the properties tab will show you a description with the available placeholders in that field.
In both cases, you may also have settings (settings tab when editing either a layout or field). Those are custom fields added to a field, which is also made available based on the configured key.