how can I use IF ELSE in variables of azure DevOps yaml pipeline with variable group?

I'm trying to assign one of 2 values to a variable in addition to variable group and can't find the reference that how to use IF ELSE.

Basically I need to convert this jerkins logic to azure DevOps.

Jenkins

if (branch = 'master') { 
   env = 'a'
} else if (branch = 'dev'){
    env ='b'
}

I found 1 reference from the following one, but this one seems to work if the variables section doesn't have variable groups.

https://stackoverflow.com/a/57532526/5862540

But in my pipeline, I already have a variable group for secrets, so I have to use name/value convention and the example doesn't work with the errors like expected a mapping or A mapping was not expected or Unexpected value 'env'

variables:
- group: my-global
- name: env
  value:
    ${{ if eq(variables['Build.SourceBranchName'], 'master') }}: 
      env: a
    ${{ if eq(variables['Build.SourceBranchName'], 'dev') }}: 
      env: b

or

variables:
- group: my-global
- name: env
  value:
    ${{ if eq(variables['Build.SourceBranchName'], 'master') }}: a
    ${{ if eq(variables['Build.SourceBranchName'], 'dev') }}: b

I think for now you're going to need to use a task to customize with name/value syntax variables and conditional variable values. It looks like the object structure for name/value syntax breaks the parsing of expressions, as you have pointed out.

For me, the following is a reasonably clean implementation, and if you want to abstract it away from the pipeline, it seems that a simple template for your many pipelines to use should satisfy the desire for a central "global" location.

variables:
  - group: FakeVarGroup
  - name: env
    value: dev

steps:
  - powershell: |
      if ($env:Build_SourceBranchName -eq 'master') {
        Write-Host ##vso[task.setvariable variable=env;isOutput=true]a
        return
      } else {
        Write-Host ##vso[task.setvariable variable=env;isOutput=true]b
      }      
    displayName: "Set Env Value"