These are similar to Blackboard tests. They can contain multiple choice questions, but also short answer questions, or questions where you need to upload a file. Depending on the question, this might be a scanned copy of handwritten work, or a typed document. For Gradescope online assignments, the file you submit does not have to be a PDF.
See the Gradescope help page: Online Assignment submissions.
You do not get an email receipt when submitting to Gradescope Online assignments.