To add custom conditions to product collection, you need to override build() function.  I hope you have a custom module with basic skeletons like registration.php and module.xml file,

Path: app/code/Magemonkeys/FilterSearch/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <!-- Magento Catalog search module's IndexBuilder.php plugin file -->
    <type name="MagentoCatalogSearchModelSearchIndexBuilder">
        <plugin name="Magemonkeys_FilterSearch::custom_filterSearchr_conditions"
                type="MagemonkeysFilterSearchPluginCatalogSearchModelSearchIndexBuilder" />
    </type>
</config>

Path: app/code/Magemonkeys/FilterSearch/Plugin/CatalogSearch/Model/Search/IndexBuilder.php

<?php
namespace MagemonkeysFilterSearchPluginCatalogSearchModelSearch;

use MagentoFrameworkSearchRequestInterface;

class IndexBuilder
{

    public function __construct(
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoCatalogModelResourceModelProductCollectionFactory $productCollectionFactory
    ) {
        $this->storeManager = $storeManager;
        $this->_productCollectionFactory = $productCollectionFactory;
    }

    public function aroundBuild(MagentoCatalogSearchModelSearchIndexBuilder $subject, callable $proceed, RequestInterface $request)
    {
        $select = $proceed($request);
        /* customization by magemonkeys */
        	$storeId = $this->storeManager->getStore()->getStoreId();
            $query = 'WHERE_CUSTOM_CONDITION';
            $select->where($query);
        /* customization by magemonkeys end here */
        return $select;
    }

}