Could someone please explain what the following lines of this code do?

This is the code that I have with me:

%let data=sashelp.cars;

    proc transpose data=&data(obs=0) out=names;
       var _all_;
       run;
    proc sql;
       select cats('_',_name_,'=missing(',_name_,');') into: stmts separated by ' ' from names;
       run;
    
    data missing;
       set &data;
       _BIGN = 1;
       /*%m_expand_varlist(data=&data,expr=cats('_',_name_,'=missing(',_name_,');'));*/
       &stmts;
       keep _:;
       run;
    proc summary data=missing;
       var _numeric_;
       output out=smry sum=;
       run;
    proc transpose data=smry(drop=_type_ _freq_) out=smry_;
       run;
   

The goal of this code is to output the number of missing values for both character and numeric variables in the data set. The code accomplishes that objective but I have difficulty in understanding the purpose of certain lines in the code.

May I know what the following part of the code does?

 select cats('_',_name_,'=missing(',_name_,');') into: stmts separated by '  ' from names;

I understand that the into part just stores the value into the macro variable stmts but what does "separated by ' ' from names" in the above line mean?

data missing;
       set &data;
       _BIGN = 1;
       &stmts;
       keep _:;
       run;

And in the above portion of the code, what is the purpose of "keep :"? What does the ":" do in that? And is the "_BIGN = 1" necessary?

And also in the final output table called smry_, I get underscores before the names of the variables. But I don't need these underscores. What can I do to remove them? When I removed the underscore after the "keep :", the underscores in the smry table went away but I was left with only 10 rows instead of 15. Help would be appreciated. Thank you.