In Cubyts, this is a two-way flow, as a Product Manager, you can assign activities and approve or reject them post reviewing. As the assignee of the activity you can mark your activity ready for review and redo the activity if requred.
Here is how the project owners can assign ownership and manage their activity review cycle
Step1 - Assign activities on the roadmap and set deadline for each activity
Image: Assigning activity to project members on roadmap
Image: Assigning dates to each activity
Step 2 - Go to your todos and click on the activities that require a review
Image: Assigning dates to each activity
Step 3 - Check all documents and based on the quality of the deliverable, approve or reject the activity
Image: Inside a document for reviewing

Image: Marking the activity as complete.
Step 4 - Add a note for the assignee in case the activity is rejected
Image: Adding a note for the assignee if the activity is rejected. This is an optional step
Image: Status on Activity Rejection
Or
Step 4 - Mark the activity as approved. Doing so will change the status of the activity to complete.
Image: Status on Activity Approval
Here is how the assignee of the activities can take the onus and ensure that their activities are completed and reviewed.
Step1 - Go to the activity you are assigned to and add your documents and artefacts
As and when you add your first document or change the status from Not Started to In Progress, system will ask you if you want to mark the activity as complete
Step 2 - Once the activity is complete, Mark it as complete.
The moment this is done, the owner of the project will get a todo to review your activity, the status of the activity will change to "In Review"
Step 3 - If the Project owner marks activity as apporved, you are good! Contunue doing the same for all other activities you are assigned to.
Or
Step 3 - If the Project owner marks it as reject, go through step 2 and mark it for review until it gets approved.