Overwrite a FlexForm from any Extension with your FlexForm in TYPO3

ext_localconf.php

<?php
defined('TYPO3_MODE') || die();

call_user_func(
    function () {
        /**
         * Overwrite FlexForm with a new one
         */
        $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['t3lib/class.t3lib_userauth.php']['postUserLookUp'][] =
            \In2code\In2studyfinderExtend\Hooks\OverwriteFlexForm::class . '->overwrite';
    }
);

FlexformStudyfinderList.xml

<T3DataStructure>
	<meta>
		<langDisable>1</langDisable>
	</meta>
	<sheets>
		<main>
			<ROOT>
				<TCEforms>
					<sheetTitle>LLL:EXT:in2studyfinder/Resources/Private/Language/locallang_db.xlf:listView</sheetTitle>
				</TCEforms>
				<type>array</type>
				<el>
					<settings.flexform.studyCourseDetailPage>
						<TCEforms>
							<exclude>1</exclude>
							<label>LLL:EXT:in2studyfinder/Resources/Private/Language/locallang_db.xlf:detailView</label>
							<config>
								<type>group</type>
								<internal_type>db</internal_type>
								<allowed>pages</allowed>
								<size>1</size>
								<maxitems>1</maxitems>
								<minitems>1</minitems>
								<show_thumbs>1</show_thumbs>
								<wizards>
									<suggest>
										<type>suggest</type>
									</suggest>
								</wizards>
							</config>
						</TCEforms>
					</settings.flexform.studyCourseDetailPage>
					<settings.flexform.select.department>
						<TCEforms>
							<label>LLL:EXT:in2studyfinder/Resources/Private/Language/locallang_db.xlf:department</label>
							<config>
								<type>select</type>
								<renderType>selectCheckBox</renderType>
								<maxitems>9999</maxitems>
								<size>5</size>
								<minitems>0</minitems>
								<foreign_table>tx_in2studyfinder_domain_model_department</foreign_table>
								<foreign_table_where>and tx_in2studyfinder_domain_model_department.sys_language_uid in (-1,0) order by tx_in2studyfinder_domain_model_department.title</foreign_table_where>
							</config>
						</TCEforms>
					</settings.flexform.select.department>
					<settings.flexform.select.graduation>
						<TCEforms>
							<label>LLL:EXT:in2studyfinder/Resources/Private/Language/locallang_db.xlf:academicDegree</label>
							<config>
								<type>select</type>
								<renderType>selectCheckBox</renderType>
								<maxitems>9999</maxitems>
								<size>5</size>
								<minitems>0</minitems>
								<foreign_table>tx_in2studyfinder_domain_model_graduation</foreign_table>
								<foreign_table_where>and tx_in2studyfinder_domain_model_graduation.sys_language_uid in (-1,0) order by tx_in2studyfinder_domain_model_graduation.title</foreign_table_where>
							</config>
						</TCEforms>
					</settings.flexform.select.graduation>
					<settings.flexform.bodytext>
						<TCEforms>
							<label>LLL:EXT:in2studyfinder_extend/Resources/Private/Language/locallang_db.xlf:flexform.bodytext</label>
							<config>
								<type>text</type>
								<default></default>
								<enableRichtext>1</enableRichtext>
								<richtextConfiguration>default</richtextConfiguration>
							</config>
							<defaultExtras>richtext[]:rte_transform[mode=ts_css]</defaultExtras>
						</TCEforms>
					</settings.flexform.bodytext>
				</el>
			</ROOT>
		</main>
	</sheets>
</T3DataStructure>

OverwriteFlexForm.php

<?php
declare(strict_types=1);
namespace In2code\In2studyfinderExtend\Hooks;

/**
 * Class OverwriteFlexForm
 */
class OverwriteFlexForm
{

    /**
     * @var string
     */
    protected $path = 'FILE:EXT:in2studyfinder_extend/Configuration/FlexForms/FlexformStudyfinderList.xml';

    /**
     * @return void
     */
    public function overwrite()
    {
        $GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds']['in2studyfinder_pi1,list']
            = $this->path;
    }
}