We are new to the platform and would need guidance on the different platforms within TTN. The difference between The Thing Network with The Thing Stack, The industrial Thing and all those. We cannot understand what the scope of each one is.
Also how can we connect a device to a public gateway.
The Things Stack is the software / stack that provides a fully featured LoRaWAN Network Server.
The Things Sandbox was known as TTN and is for learning & community use.
The Things Industries is the creator of TTS and runs paid for instances for commercial use which is fully described on the TTI home page.
To add a device you can read the documentation. A device does not connect to a gateway, it broadcasts and it if is heard by a public gateway and is registered on TTN then the message will be passed on. Forum search explains this many many times. Generally when getting started you will need your own gateway as that is the only way you can be sure of a device being heard. All of this is covered in the Learn section which we assume that you have read as it covers the basics. Please make use of the documentation and use the forum search as well.
As above, please stop asking me direct questions on Slack - if you want assistance for free, do it here or publicly.
Otherwise purchase a support contract from TTI.