Craft 3 version - How to get entries related to current category only (not children) I need some help with syntax on this for Craft 3, please.

I want to use this piece of code: 

{% set fruitEntries =  craft.entries.relatedTo({ relatedTo: catP,id: 'and, not '~str }) %}


from this answer here:

How to get entries related to current category only (not children)

It's exactly what I need to do but I'm not sure how to code it for Craft 3 so that I can pull in entries from JUST my 'products' section:

craft.entries.section('products')...


Thanks in advance.



UPDATED INFO.

Category Group = Products

Entry Sections using these categories = Products &amp; Case Studies

My category structure is as follows:

Categories are:


Travelling Irrigators


Superseeded Models

Hydrants
Filters
Pontoons
Pumps


I'm trying to return just the product entries (not case studies) under 'Travelling Irrigators' but not including the 'Superseeded Models'.

The other post does exactly what I need but it returns the case studies too.
These two pages should help you from the Craft 3 docs

https://docs.craftcms.com/v3/dev/element-queries/entry-queries.html#relatedto 

https://docs.craftcms.com/v3/relations.html