So To my knowledge, as @Andra_Buica mentioned, AI Center has on-prem model. Most of the organization prefer to keep the secured data within the network, so they go for on-prem model.
@Lucas_Ferraz - For your question, for any document extraction that does not require the Form extractors or ML Extractor skills you can go without API keys (Document understanding keys). if you are using Digitization with Google ocr + Regex extractors, you would not require Ai Center.
@melanie - The difference between airgapped and on-prem connect is basically the internet connection. Airgapped servers will not have internet connections, everything has to be downloaded and installed. where as, on-prem connect will pull up the installation files over the web/cloud. Hope this helps.
@Andra_Buica - Nice to meet you here