@BertP -
1. Do most people develop their unmanaged solutions in a sandbox environment
As you're aware, typically, the following pattern will be used for ALM per department:
(N x) DEV-ENV (Developer Plan) -> (1 x) UAT-ENV (Sandbox) -> (1 x) PROD-ENV (Production)
Technically, there are two types of Developer environments.
- A Sandbox environment which can be used either for Development or UAT.
- A Developer Plan environment
Your organisation could create two Sandbox environments (1 for Dev and 1 for UAT), and then a Production environment.
However, organisations nominally opt for users to use the Developer Plan environment for Dev. This environment can be provisioned by anyone in your tenancy themselves and comes with all the premium features.
The reason for this is usually because of logistics, DB space (which is shared between all instances in the tenancy) and costs.
Setting up a DEV environment via the Developer Plan is simply a case of navigating to:
https://powerapps.microsoft.com/en-gb/developerplan/
Please note as of today, not all web resources included in a "full fat" environment like a Sandbox or Prod are included in a Developer Plan environment.
Further reading:
https://learn.microsoft.com/en-us/power-platform/admin/environments-overview
2. Does this prevent emails of flows or app connectors going out to users?
The flow will run in the context of the Environment it is triggered from. Normally you would turn off flows in the Dev or UAT environment and turn them on in the Production environment. Note that turning on a flow in a managed solution will not create an unmanaged layer in that solution.
Further reading on Solution Layers: https://learn.microsoft.com/en-us/power-platform/admin/environments-overview
3. Moving an Unmanaged Solution in a Production Environment over to a Sandbox environment.
I would import the Unmanaged Solution in your Production environment over to your Developer Plan environment, and then delete the Unmanaged Solution in your Production environment.
Note that data entered in one environment will not be imported over into another environment. Importing a solution will include the table metadata for example but not the data inside the tables. You will need to perform a data import exercise using the Import, Import from Excel, Dataflows, or the Dynamics 365 wizard (the latter I have written a blog on here:
https://powerusers.microsoft.com/t5/Power-Apps-Community-Blog/Bulk-update-Dataverse-LookUp-column-us...
There are also third party tools like XRMToolBox: https://www.xrmtoolbox.com/plugins/Colso.Xrm.DataTransporter/