if object_id('tempdb.dbo.#tmp') is not null drop table #tmp;

create table #tmp (id int);

declare @sql varchar(2000);
set @sql = 'insert into #tmp(id) values (1);';
exec (@sql);

select * from #tmp;

id
-----------
1

Declare @strSql varchar(2000);
Set @strSql = '';
Set @strSql = @strSql + 'Create table #tmp (id int);';
Set @strSql = @strSql + 'Insert into #tmp(id) values (1);';
Set @strSql = @strSql + 'Select * From #tmp;';
Exec (@strSql);

Declare @strSql varchar(2000)
Set @strSql = ''
Set @strSql = @strSql + 'Create table ##tmp (id int)'  -- Now "##tmp', not "#tmp'
Set @strSql = @strSql + 'Insert into ##tmp(id) values (1)'
Exec (@strSql)
Select * from ##tmp
drop table ##tmp