System.xml guide in Magento 2

Create a file at etc/adminhtml/system.xml for that specific module.

In the system.xml file, it is possible to define four different types of entities those are below

Create a tab:

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
<system>
<tab id="A_UNIQUE_ID" translate="label" class="a-custom-css-class-to-style-this-tab" sortOrder="10">
<label>A meaningful label</label>
</tab>
</system>
</config>

Only id is required

Create a Section:

<section id="A_UNIQUE_SECTION_ID" showInDefault="1" showInWebsite="0" showInStore="1" sortOrder="10" translate="label">
<label>A meaningful section label</label>
<tab>A_UNIQUE_ID</tab>
<resource>VENDOR_MODULE::path_to_the_acl_resource</resource>
</section>
</system>
</config>

ID is required

Create a Group:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
<system>
<tab id="A_UNIQUE_ID" translate="label" class="a-custom-css-class-to-style-this-tab" sortOrder="10">
<label>A meaningful label</label>
</tab>

<group id="A_UNIQUE_GROUP_ID" translate="label comment" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="1">
<label>A meaningful group label</label>
<comment>An additional comment helping users to understand the effect when configuring the fields defined in this group.</comment>

</group>
</section>
</system>
</config>

Create a Field:

<field id="export" translate="label" type="Magento\OfflineShipping\Block\Adminhtml\Form\Field\Export" sortOrder="5" showInDefault="0" showInWebsite="1" showInStore="0">
<label>Export</label>
</field>