According to the PhpSpreadsheet Doc it's neccessary to install it with composer. In my case I just have a webspace without Terminal but Plesk. Is it anyway possible to use PhpSpreadsheet, like it is with PHPExcel where you just have to place the files in any location? What do I have to do to get it run? I found no further information how to with only FTP webserver access.
In your case there are two options for you!
Answer: 1
Alternative method without terminal
Run composer with a PHP script in browser
Answer: 2
Third party sites, which allow to download composer packages online. get PHPspreadsheet latest version.
https://php-download.com/package/phpoffice/phpspreadsheet
Bonus You can download almost any composer packages @ https://php-download.com