While Adobe does have some managed services capabilities, you can expect to be directed to its partner network. Adobe will sell you the software license, but (as of this morning) they have 431 Solution Partners: https://solutionpartners.adobe.com/s/directory/?solution=commerce
Many of these agencies, SIs, and dev teams work with both Adobe Commerce and Magento Open Source, although a limited number work more exclusively with Adobe Commerce.
You can learn about some of the Managed Services that Adobe offers, such as code audits, at https://business.adobe.com/products/magento/fully-managed-service.html
Whether you use the Magento Open Source version or the Adobe Commerce version, you'll need developers to help with ongoing patches, upgrades, and maintenance, as well as changes and improvements, such as adding new integrations and extensions. From my experience, if you won't have internal resources to manage and complete this work, you're going to select an agency to assist with this. There are a lot of great ones.
Best of luck!
