The SAP system first takes the parameters from the kernel, then updates them with those in default profile and then with those in the instance profile. Not all parameters can be changed by the admins as they are defined in the kernel code.
The default profile and the instance profiles are stored in /usr/sap/SID/SYS/profile (and /sapmnt/SID/profile).
To see the list of SAP parameters, run the report RSPFPAR or RSPARAM using SE37. The reports show the parameters for the instance from where you are running the reports. You can use TU02 to display the parameters for all the instances.
You can display the parameters from OS using the following commands
sappfpar name=<SID> nr=<nn>
sappfpar is useful when you want to verfy the paramters while the SAP system is down.
Some parameters can be changed temporarily and dynamically using the transaction RZ11. Use RZ10 to change the parameters permanently.