I'm having a lot of issues trying to implement a native ios library into a flutter plugin, this is the plugin podspec:

Pod::Spec.new do |s|
    s.name                = 'flpl'
    s.version             = '0.0.1'
    s.summary             = 'A new flutter plugin project.'
    s.homepage            = 'http://example.com'
    s.license             = { :text => 'License Text' }
    s.author              = { 'Example' => 'example@example.com' }
    s.source              = { :path => '.' }
    s.source_files        = 'Classes/**/*'
    s.public_header_files = 'Classes/**/*.h'
    
    s.dependency 'Flutter'
    s.platform = :ios, '12.0'
  
    s.subspec 'vLib' do |ss|
      ss.preserve_paths = 'libVendored.a'
      ss.ios.vendored_libraries = 'libVendored.a'
      ss.xcconfig = { 'OTHER_LDFLAGS': "-ObjC", 'ENABLE_BITCODE' => 'NO' }
    end
    
    s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386' }
end

Note: iOS 13 was chosen because it gave the least amount of compile-time errors, not for a specific reason

This is the generalized file structure:

android
example
 ↳ ios
    ↳ Podfile
lib
ios
 ↳ Assetts
 ↳ Classes
    ↳ vLib
       ↳ someHeaderFile.h
    ↳ flplPlugin.h
    ↳ flplPlugin.m
 ↳ libVendored.a
 ↳ flpl.podspec

flplPlugin.h: no difference from what flutter creates.
flplPlugin.m:

#import "FlplPlugin.h"
#import "someHeaderFile.h"
 
@implementation FlplPlugin
+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar
{
  FlutterMethodChannel* channel = [FlutterMethodChannel methodChannelWithName:@"flpl" binaryMessenger:[registrar messenger]];
  FlplPlugin* instance = [[FlplPlugin alloc] init];
  [registrar addMethodCallDelegate:instance channel:channel];
}
 
- (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result
{
  if ([@"ping" isEqualToString:call.method])
  {
    result(@"pong");
  }
  else if ([@"auth" isEqualToString:call.method])
  {
    --=*( call to someHeaderFile class functions )*=--
  }
}
 
@end

When I try to build the plugin's example app (flutter clean; flutter build ios inside the example folder) xcode errors with the following:

  ld: warning: object file (/path_2_plugin/example/build/ios/Release-iphoneos/flpl/libVendored.a(FlplPlugin.o)) was built for newer iOS version (12.0) than being linked (9.0)
  ld: warning: object file (/path_2_plugin/example/build/ios/Release-iphoneos/flpl/libVendored.a(flpl-dummy.o)) was built for newer iOS version (12.0) than being linked (9.0)
  Undefined symbols for architecture arm64:
    "_OBJC_CLASS_$_SomeHeaderFileClass", referenced from:
        objc-class-ref in libVendored.a(FlplPlugin.o)
  ld: symbol(s) not found for architecture arm64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  
  ld: warning: ignoring file /path_2_plugin/example/build/ios/Release-iphoneos/flpl/libVendored.a, building for iOS-armv7 but attempting to link with file built for iOS-arm64
  Undefined symbols for architecture armv7:
    "_OBJC_CLASS_$_FlplPlugin", referenced from:
        objc-class-ref in GeneratedPluginRegistrant.o
  ld: symbol(s) not found for architecture armv7
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Error (Xcode): Undefined symbol: _OBJC_CLASS_$_SomeHeaderFileClass
Error (Xcode): Undefined symbol: _OBJC_CLASS_$_FlplPlugin
Encountered error while building for device.

Looking into the library though, I found it: nm -g /path_2_plugin/.../flpl/libVendored.a | grep '_OBJC_CLASS_$_SomeHeaderFileClass' outputs: U _OBJC_CLASS_$_SomeHeaderFileClass

Flutter doctor:

[✓] Flutter (Channel stable, 2.10.2, on macOS 12.1 21C52 darwin-arm, locale it)
[✓] Xcode - develop for iOS and macOS (Xcode 13.2.1)

Also tried on a non M1 Mac but had the same errors. Non-M1 Flutter doctor:

[✓] Flutter (Channel stable, 2.10.2, on macOS 11.4 20F71 darwin-x64, locale it-IT)
[!] Xcode - develop for iOS and macOS (Xcode 12.5.1)

(i currently cannot upgrade xcode on the non m1 mac)