Assuming you have a repo with the mulitple solutions in different paths like those in the image below;

Based on your requriment, you may run git diff command to know which path the change commit is pushed into, so as to determine whether to build those solutions or not.

Here is a sample YAML pipeline for your reference.

parameters:
- name: SLNs
  type: object
  default:
  - Solution1
  - Solution2

stages:
- stage: CI
  jobs:
  - job: Build
    steps:
    - checkout: self
      fetchDepth: 0 # Set fetchDepth as 0 to disable default shallowfetch and make sure git diff command works

    - ${{ each SLN in parameters.SLNs }}:
      - powershell: |
          Write-Host "Build.SourceVersion is $(Build.SourceVersion)"

          # Check if there are changes in ${{SLN}}
          if (git diff --name-only $(Build.SourceVersion)^ $(Build.SourceVersion) -- ${{SLN}}) {
              Write-Host "Changes detected in ${{SLN}}"
              Write-Host "##vso[task.setvariable variable=build${{SLN}}]True"
          }
          else {
            Write-Host "No changes detected in ${{SLN}}"
          }
        displayName: 'Determine to Build ${{SLN}}'
      - task: DotNetCoreCLI@2
        inputs:
          command: 'build'
          projects: '${{SLN}}/*.sln'
          arguments: '-o:$(Build.BinariesDirectory)'
        displayName: 'Build ${{SLN}}'
        condition: eq(variables['build${{SLN}}'], 'True')
    - task: CopyFiles@2
      inputs:
        SourceFolder: '$(Build.BinariesDirectory)'
        Contents: '*.dll'
        TargetFolder: '$(Build.ArtifactStagingDirectory)'
      displayName: Copy .dll files 
    - publish: $(Build.ArtifactStagingDirectory)
      artifact: drop

- stage: CD
  jobs:
  - deployment: Deploy
    environment: E-Test
    strategy:
      runOnce:
        deploy:
          steps:
          - script: |
              tree $(Pipeline.Workspace) /F /A
            displayName: Check artifacts

However, it may NOT be reliable if there are multiple commits under different paths within one single push. Therefore, it is still suggested creating seperate pipelines to be triggered by specific Paths filter and procceed to build and publish the solution in that path.