Hello @Wong, David ,
as with every system, designing the architecture of IoT platforms is not simple.
IoT touches both the cloud and the physical world and it must bring value in an innovative way.
It would help if you have insights regarding eg. personas, use cases, requirements (both functional and non-functional), hot, warm, and cold path estimations, communication channels, possible use of edge technology, multi-tenancy, and level of open source resources, etc.
Also, the skillset of the team is important. Normally, this will tell you to go for a SaaS, APaaS (like IoT Central), or PaaS solution.
I recommend thinking big but acting small.
First, get familiar with Azure IoT using the training modules available at MS Learn. Here is a good collection to get started.
Then, check the Azure IoT Reference architecture. You do not have to implement this completely right away but is gives you good pointers for the future.
From there, select a couple of proof of concept projects, max 3, and all needing different solutions/techniques. This will give you good insights into the challenges you face. It will also tell you how to measure the value the solutions bring to you.
From there, do a pilot before pro-production and finally scaling up in production.
I understand this is not a simple answer and it takes some effort from your side.
Feel free to come back to this forum to discuss detailed questions along the way.
Check out the recordings of the recent Azure IoT Architecture Summit too. The July IoT Series can help you too.