In your db_schema.xml file which should be inside Magesan/Extension/etc/db_schema.xml

<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">
    <table name="tableName" resource="default" engine="innodb" comment="Table Comment">
        <column xsi:type="boolean" name="columnName" nullable="false" comment="column comment" />
    </table>
</schema>

Save the file.

You just only need to fire php bin/magento setup:upgrade command and you will see the column will added to table.

Happy Coding…