You can use system stored procedure sp_msloginmappings to list out the SQL logins and database user mappings.

Syntax
sp_msloginmappings @Loginname , @Flags

@Loginname – Optional argument, in case if you not specify the Login name procedure will return the result for all the SQL Server logins
@Flags – You can specify value 0 or 1, 0 value will show user mapping to all databases and 1 will show the user mapping to current database only. Default value is 0

use master
go
exec sp_msloginmappings 'sa', 0

use master
go
exec sp_msloginmappings 'sa', 1

In case you want to run the sp_msloginmappings across multiple SQL Instance using either Central management server or powershell. You can use the below script.

 create table #loginmappings(  
  LoginName  nvarchar(128) NULL,  
  DBName     nvarchar(128) NULL,  
  UserName   nvarchar(128) NULL,  
  AliasName  nvarchar(128) NULL  
 )  
 
 insert into #loginmappings
 EXEC master..sp_msloginmappings
 
 select * from #loginmappings
 
 drop table #loginmappings