It depends on the concurrent requests you are running. A pl/sql stored procedure will run exclusively on the dbTier. ASCP 64-bit planning executables run on the appTier and I have seen them consume more than 30 GB of RAM. In general, greater than 90% of the concurrent workload happens on the dbTier.
Some sites also have a concurrent tier in addition to the database and application tier. But as already stated, in general, you'll tend to be running things on the database tier because ultimately the work a concurrent request tends to be doing is on the database. The number of standard requests which do things on the app side instead is pretty limited I believe.
As mentioned above, its more of Database side. However concurrent manager processes (OPP & FNDLIBR) require memory(RAM) and CPU ; depends of the configuration (Heap Size) and other things.