An SQLCLR function is part of an assembly. An assembly can contain multiple functions. It can (in theory) be made up of multiple files, although that is unusual.

You can get the actual DLL files with this

SELECT
  a.name AssemblyName,
  f.name FileName,
  f.content Dll
FROM sys.assemblies a
JOIN sys.assembly_files f ON f.assembly_id = a.assembly_id
-- if you want to filter to certain SQLCLR functions
/*
WHERE a.assembly_id IN (
    SELECT *
    FROM sys.assembly_modules m
    WHERE m.object_id = OBJECT_ID(N'YourFunction')
)
*/

You can de-compile the DLLs with your favorite .NET decompiler.

If you want to just transfer them, you can commit them to source control as CREATE ASSEMBLY FROM followed by the binary data

Making Charlieface's second recommendation more explicit, just transfer the assembly. You can do this through SSMS by choosing the assembly in Object Explorer and selecting "Script Assembly as..." from the context menu. Of course, since that just uses SMO on the back end, you can also do this with a little scripting. Here's a quick-and-dirty powershell script ( Get-DbaDatabase coming from the always delightful dbatools module).

$db = Get-DbaDatabase -SqlInstance yourServer -Database yourDB;
foreach ($a in ($db.Assemblies | where IsSystemObject -eq $false)){
    $a.Script();
}