Why all vendor folder go to app/code?
解决方案
Please download from the downloads section of the website. Or you can use composer
composer create-project --prefer-source --repository-url=https://repo.magento.com/ magento/project-community-edition=2.3.2 ./
Where 2.3.2 is magento version.
Do not download from git
Git version is for magento core development if you plan to submit pull requests to the magento git repo