Допустим я выполнил какой то проект. Как мне завершить его, не потеряв задачи в нем и все прочее?
Unfortunately, it's not possible to actually complete a project, but with Todoist Premium, you would have an option to "archive" it. Right-click on a project, choose "Archive" and it will be removed from the list without being deleted. You can then access archived projects and their tasks through a link below your project list.