cd some/directory
sfdx auth:web:login -a my@username.dev --instanceurl=https://my-sfdc-instance.my.salesforce.com
sfdx force:project:create -n myProject --manifest
cd myProject
sfdx config:set defaultusername=my@username.dev
# Modify the package xml to retieve whatever we want
sfdx force:source:retrieve -x manifest/package.xml
# or retrieve this way
sfdx force:source:retrieve -m "ApexClass:MyApexClass"
      
    

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