The difference between SQL left join main table restrictions written after on and where

As the title , Suppose there is such a sql topic

with a as (
select 1 as id , 'a' as name 
union all
select 1,'b'
),
b as  (
select 1 as id1, 'q' as name1
union all 
select 1 , 'w'

)

select count(*) from (
select *,'a' as num
from a 
left join b 
on a.id = b.id1 
and  a.name = 'a'

Ask the answer ？

Presumably most people will answer 2 , At least I am . In my original understanding , This sql The execution sequence of should be like this

1. Perform correlation , There are four results , The data is 1 a 1 q; 1 a 1 w; 1 b 1 q; 1 b 1 w;
2. filtering , Get rid of a.name <>'a’ Of ,
   The final result is 1 a 1 q; 1 a 1 w;
   But this is wrong ！！！

For this sql Come on , The order of execution should be like this .

1. First detected left join , Go straight back to a Table all results , The result is 1 a ;1 b
2. take a Table compliance a.name = 'a’ Of , To be with b Tables are associated , Do not meet the conditions ,b Table set as null, The result is 1 a 1 q; 1 a 1 w; 1 b null null

therefore , The result is 3
Make a note of , To sum up your experience