Sub-processors (processors) are vendors that process personal data on behalf of the controller or a main processor — e.g. hosting, database, email, monitoring. The concrete list depends on your platform contract and should be provided publicly or contractually.
Infrastructure to run the web application and store data in the cloud. Region and sub-processing should be governed in the data processing agreement.
Services for transactional email (invitations, password reset, notifications) may process personal data (e.g. email address).
For each service, review: purpose, categories of personal data, third-country aspects, sub-processing, and deletion concept. Use only contractually bound providers.
Your practice and the platform provider should have a data processing agreement (Art. 28 GDPR); the provider in turn concludes DPAs with its sub-processors. Observe documentation duties and instructions rights.