Manual testing should be performed by a combination of internal and external resources. The availability of resources and cost should be considered when determining the proportion of internal vs external testers. A common approach is to use external testers to find bugs and validate results, while internal testers validate the outcome of tests to ensure they align with business processes. It is important to also invest in developing internal testing resources to validate external results and avoid risks. Regression testing is an area where external testers can provide added value.
More to the question of “Who should perform the manual testing?” Wim Coremans replies in following video: