As the documentation says, "collections are essentially just folders", so why can't I search for them by name? I have a couple of hundreds of collections its name represent theme of bookmark to organize them.

A while back, hadn't been happening recently in my experience.. Sometimes when you select a collection from autocomplete drop-down, the page loads with a [collection:] operator appended to searchbar with an obscure long train of characters that seems to be that collections id.

Adding an operator, search-style, to that didn't do anything 😢

I completely agree. We need a new "collection" search operator.

Adding a "-" in front of this operator would then exclude the collection from the search.

We could use this operator multiple times in the search query in order to include/exclude multiple collections.

Using this operator would search in and only in the collection specified.

In order to include nested sub-collections in the search, we could specify a "nested" operator. Another alternative for that would be to append a special character like a "+" after the collection name (but then what if the collection has this character is its name?, the collection name would have to be surrounded with double quotes, etc) I think it would be better to append this character after the operator itself: "collection+", which would be synonym of "collection", would include sub-collections; "collection-" would exclude nested collections.

To go the extra mile with this idea, it could be extended to specify the number of hierarchy levels of nested collections to include, like "collection2" or "collection+2" would include only the sub (1st level) and sub-sub collections (2nd level) but not the sub-sub-sub (3rd level) and further of nested collections. "collection0" would be synomym of "collection".

+1 to this. I read through the documentation for search syntax and it seems this is not supported yet.