You cannot get at "underlying code" at the application-encoding level, but, if you have a Full Access account, you can open the file with the ability to see all of the object-based FM-native scripts, table and field definitions, layout structures, etc.
You must have a Full Access account to do this. Many FM files are set to open without requiring a password, but many of those files open with lower-level permissions than are needed to get into the guts of the beast.
If you know a Full Access password, but are not being offered anywhere to log in, Open the file while holding down the Shift Key (windows) or the Option key (Mac). This will force the login screen to appear for you to enter your Full Access password instead of opening with limited permissions.
If you don't have Full Access permission when the file opens, you usual cannot see how anything works.
FileMaker's support for version 5 is no longer available. I believe the oldest version still officially supported is FileMaker 9, which is now several years old. If the file is worth keeping (after you look it over) consider migrating it to a current version.