Saas was the original way of hosting databases and making them available to clients. The reason why we cannot develop in Python is because multiple user share the same server and to load Python we need to restart it, which we cannot do to avoid disturbing the other clients.

Odoo recently introduced SH which is a way of keeping each client on its own server. This is why we can develop in Python on SH. SH is divided into three sections, the production branch, the staging one and the development section.

1. Production: this is the live db of the client where they work. We must touch this as little as possible and with care. In case of an error happening in the production branch, it will roll back to the last working version
2. Staging: This is a copy of the production branch and where we test the developments before pushing them to production, there might be multiple staging branches
3. Development: This is where we create our branches and develop.

Anything that can be created in Python can be created in XML too. Depending on what you are trying to create, you can usually define anything like this

<record id="view_id" model="model_name">
  fields list (you can find them inside the model in python)  
</record>