For the first task listed (having dependent responses in second combo box), you would want to create a predefined list in sharepoint, that has a list of the project titles, and sub task name in a second column, like so:
Project Tasks Associated
Make app Make Sharepoint List
Make app Link Sharepoint to app datasource
Eat lunch Buy Food
Eat lunch Cook Food
Next you would link that new list as a data source in power apps, as you will be using these for your combo box list sources.
For the first combo box, put this for it's Items control:
Distinct('ProjectListSharepointName', Project)
For the second box put this for its Items control:
Distinct(Filter('ProjectListSharepointName', Project = combobox1.Selected.Value),'Tasks Associated')
That will get you the dependent box.
With power apps, you would link it to a pre existing list, the information gathered in the app would make a single record/ row in that list.
For the timesheets you were looking to create, I think the best way to go would be to create a calculated column in the apps output list that forecast time needed based on the input value? Sorry, not 100% sure what the desired outcome was for you.
