Yeah, I read it. It says: "It is a multi-service architecture where several software components interact with each other. This choice was made to ensure the modularity of the code and to guarantee the exploitability of certain OSRD services by external applications"
Modularity of code is a common explanation for using a multi-service architecture, but it's not a particularly strong one (services are just one way of modularizing code). In terms of OSRD services being accessed by external applications, it doesn't seem like that would work particularly well, given the coupling between the pieces. The only pieces that would make any sense to me for outside applications to directly in would be the PostgreSQL database... and if that were the plan, I'd just host the whole thing in PostgreSQL.
If you read deeper, you'd find out that for example the simulation engine is a separate (older?) system written in Java that does have an API but does not implement other components like designing the railroad system etc.
Modularity of code is a common explanation for using a multi-service architecture, but it's not a particularly strong one (services are just one way of modularizing code). In terms of OSRD services being accessed by external applications, it doesn't seem like that would work particularly well, given the coupling between the pieces. The only pieces that would make any sense to me for outside applications to directly in would be the PostgreSQL database... and if that were the plan, I'd just host the whole thing in PostgreSQL.