From what I understand (I'm sure more knowledgeable community members will jump in if anything is incorrect) you have two options: Mounting the namespace to your workspace, or importing and exporting individual files from your workspace. 

Option 1:

From the Servers tab, which your screenshot shows, you can hover over any namespace to find options to mount the namespace to your VS Code workspace, either in a read-only or editable session. This screenshot shows it with the buttons - pencil is editable, eye symbol is read-only. 

Option 2:

My preferred usage method is to use the Explorer Panel below the servers panel. To start, click choose server and namespace to sign in to your chosen namespace: 

This will connect your workspace to that namespace. After this you can see the classes in your namespace. 

If you would like to edit classes, right click on them and choose Export:
 

After this you can use vs code as normal, with a copy of the file on your local computer. If you want to create a new class, just create a new file in your VS Code workspace with the .cls extension, and then it will auto-compile on save. If it doesn't, you can right click on the file from the menu and choose Import and Compile.

The connection info is stored in .vscode/settings.json so you can always edit it here, or delete it and reconnect to a different namespace. 

You don't need to obtain an evaluation license directly, you just need to download the community edition of InterSystems IRIS for Health (IRIS For Health includes FHIR server capabilities). You can download installation kits here: Evaluate InterSystems Products or containers from docker hub with: 

docker run --name my-iris-health --publish 52773:52773 --publish 1972:1972 -d intersystems/irishealth-community:2025.3

The community edition is free to use, but has limits to prevent it being in production (e.g. max database size, max number of connections etc). 

Easiest way is with the community edition in a local docker container. Install docker and then run: 
 

docker run --publish 52773:52773 --publish 1972:1972 intersystems/iris-community:2025.3

You can also check out some of the dev templates which put this in a docker-compose file so you can clone a repo and run a working version with example code with just a couple of commands. You just have to make sure that in the Dockerfile it is pulling from version 2025.3 (or latest-cd). 

- ObjectScript Template

- Interoperability template

- Embedded Python Template
(theres more available on the open exchange)