Try below steps

**Step:1** Create `default.xml`at location `app\code\<Vendor-Name>\<Module-Name>\view\frontend\layout` and add below code

    <referenceContainer name="page.top">
    	<block class="Magento\Framework\View\Element\Template" name="custom-block-name" template="<Vendor-Name>_<Module-Name>::page\custom.phtml"/>
    </referenceContainer>
    
**OR**

     <referenceContainer name="main.content">
    	<block class="Magento\Framework\View\Element\Template" name="custom-block-name" template="<Vendor-Name>_<Module-Name>::page\custom.phtml"/>
    </referenceContainer>

**Step:2** Create `custom.phtml` at location `app\code\<Vendor-Name>\<Module-Name>\view\frontend\templates\page\` and add your code

for eg.

    <?php 
         echo "Custom Block";
    ?>

**Step:3** After run compile command  

    php bin/magento setup:di:compile