When designing a cloud application, there are many options. Even if you have settled on a cloud provider, the number of services from which to choose can be overwhelming. It helps if you group cloud services into three categories: Software as a Service (SAAS), Platform as a Service (SAAS), and Infrastructure as a Service (SAAS). There may be some overlap between these services but thinking of them in this way can simplify your options.
Software as a Service (SAAS)
Software as a Service (or "SAAS" for short) describes services that are pre-built. You can sign up for them and start using them immediately with little or no configuration. Examples include email services, such as Gmail and Customer Relationship Management systems, such as Microsoft CRM. Many of these systems allow you to customize them, but that is your choice. If they match your needs, you can sign up and start using them very quickly.
These are the simplest services to use, as they require the least amount of custom code.
Platform as a Service (PAAS)
Platform as a Service (or "PAAS" for short) describes services you can consume or build on within your applications. They provide some basic functionality, such as a website, a web service, a database, or machine learning models. But they require some custom code on your part to make them useful. These services free you from implementing part of the application, so you can focus on the business logic.
For example, if you want to implement custom vision recognition in your software, you could create and train your own model or you could simply call the Custom Vision API of Microsoft Cognitive Services. The latter is much simpler because much of the data collection and compute processing has been done for you to create an existing model. You can then take the results of that model, use it to identify objects in a photo or video, and build the business logic of your application with that identification information.
Infrastructure as a Service (IAAS) refers to software components that are pre-built
Infrastructure as a Service (or "IAAS" for short) describes the deployment of your data and code to a virtual machine or a container in the cloud. With this category, you are responsible for all the code and data, but the networking, infrastructure, and hardware are abstracted away from you. Those things are handled by the cloud provider automatically.
The final category used to be our only option. In years past, we had to buy, install, configure, and maintain our own hardware. In addition, we were responsible for installing and patching the operating system and any packaged software we are using. Each of the cloud solutions described above takes that responsibility away from us.
How to decide
As a general rule, it is more cost-effective to buy or rent a solution that fits your needs than to build that system yourself. This may be offset by the amount of customization you need to do, so take this into account when you choose your services.
We can think of the categories above as existing on a continuum, as shown below:
SAAS -> PAAS -> IAAS -> On-Premises
As we move from left to right along this continuum, we gain more control over our system; but we are required to do more of the work ourselves. Doing the work ourselves tends to be expensive because we cannot share that development cost among many customers, as a cloud provider can. On the other hand, we may require flexibility that is not offered by a given service, so we may need to move to the right and absorb that cost to gain that flexibility.
My recommendation when creating an application or service is to begin on the left side of the continuum above and consider if a service in that category will meet your needs. If it does, then stop and consider using that service. If not, move to the right and consider if the next category meets your needs, and so on.
If you can find a SAAS service that you can use, you should strongly consider that. It may be that no such service exists or that all are prohibitively expensive. If that is the case, look at PAAS services and try to find one or more that will meet your needs. If no such service exists, you may need to write your own. In this case, it may be possible to deploy your solution to a different PAAS service (Microsoft App Services and Microsoft CosmosDB, for example). However, if no PAAS service meets your needs, you may need to install and deploy everything yourself. The cloud can still help as you can deploy to IAAS services, using Virtual Machines and containers. You are still freed from maintaining the hardware and underlying infrastructure of the network.
Finally, there may be reasons you cannot deploy your application or data to the cloud. Accessing data or services across the Internet may result in unacceptable latency for your application or local laws may require you to store data in a country in which no cloud data center exists. In these cases, you can purchase and maintain your own servers in your own data center (or use a non-cloud data center). This requires the most work on your part and tends to be the most expensive option, which is why so many companies are moving to the cloud.
As you can see by the above example, there are no answers that fit every scenario. You need to consider your needs, the available cloud services, and each category.
Keep in mind that you can split your application into multiple services and deploy some as SAAS, some as PAAS, and some as IAAS. Doing this requires that your application is divided into component services that can be easily split.
If that is not the case, be aware that you can change your cloud deployment over time. For example, you may choose initially to "lift and shift" an on-premises application - that is, move it from a local server to a Virtual Machine in the cloud. Over time, you can refactor your code, dividing your application into smaller services that can be deployed as PAAS services. This should reduce the amount of code you need to maintain and reduce your costs.
Generally speaking, you should avoid "reinventing the wheel". Take advantage of work that has already been done. And take advantage of services that do things not part of your core business. Every hour you spend patching an O/S, installing a software update, and replacing hardware is an hour that you are not spending enhancing and refining your core business logic.