Use the 
this.template.querySelector('lightning-datatable').getSelectedRows() 
to retrieve the selected rows.



<template>

<lightning-button onclick={processSelected} label="GetSelection"> 
</lightning-button>
           <lightning-datatable
                    if:true={showProducts}
                    key-field="id"
                    data={products}
                    columns={columns}>
    </lightning-datatable>
</template>



import {LightningElement, wire} from 'lwc';

const columns = [
    { label: 'Product Name', fieldName: 'Name', type: 'text' },
    { label: 'IsActive', fieldName: 'IsActive' ,type: 'boolean'},
    { label: 'Product Code', fieldName: 'ProductCode', type: 'text' },
    { label: 'Product Family', fieldName: 'Family' , type: 'text'},
];

export default class CategoryProductAssociation extends LightningElement {
    products = [];
    columns = columns;
    searchKey;
    showProducts = false;
    categoryid;


    processSelected(){
        console.log("getSelectedRows => ", 
        this.template.querySelector('lightning-datatable').getSelectedRows());
    }

}