The Build SPI architecture diagram describes the component architecture with data/control flows between the stakeholders ALM, client, and build provider (3rd party). Core component is the build system located in the ALM system. The build system provides the Build API that provides abstract build services to other ALM components and controls the build provider component at the 3rd party system.
The build provider is a "proxy" component that controls the actual build environment located at the build provider system(s). The build provider component implements the SPI API and may be provided by the 3rd party but can also be provided in a proxy model by a 4th party (or RH as part of the ALM offering).
The ALM UI instance running in the users browser only communicates with the ALM systems and the build service API. On exception is the aquisition of the build provider credentials for accessing the build provider services. For the current ALM system, this will be implemented by an OAuth flow that directly communicates with the build environment (not the provider as it may not be located in the responsibility sphere of the 3rd party system), aquires the OAuth token and submits it to the ALM build system via the build service API. The build service then uses that token to access the build provider and the build environment.