BigQuery query review for session based attribution

Question

I've been struggling for a while to get a query to return the number of sessions and users per source/campaign/medium/content, based on the current session of the user and not first touch based attribution.

Will someone review this? As a side question, why do I have to CONCAT session id and user id to get accurate session count?

---tempquery

with first_page_view as (
select
event_date,
user_pseudo_id,
    (select value.int_value from unnest(event_params) where key = 'ga_session_id') as session_id,
    ((select value.string_value from unnest(event_params) where key = 'medium' )) as session_medium,
   ((select value.string_value from unnest(event_params) where key = 'source')) as session_source,
    ((select value.string_value from unnest(event_params) where key = 'campaign')) as session_campaign,
    ((select value.string_value from unnest(event_params) where key = 'content')) as session_content,
    ((select value.int_value from unnest(event_params) where key = 'entrances' )) as entrances,
   
from
`*******.events_*`
where event_name = "page_view"
)
,

conversion_event as (
select

user_pseudo_id,
(select value.int_value from unnest(event_params) where key = 'ga_session_id') as session_id,
from
`ga4*****************.events_*`

where event_name = 'ConversionEvent'

)
--mainquery
SELECT
a.event_date,
a.session_source,
a.session_medium,
a.session_campaign,
a.session_content,
count(distinct (CONCAT(a.session_id, a.user_pseudo_id))) as sessions,
count(distinct a.user_pseudo_id) as users,
count(distinct (CONCAT(b.session_id, b.user_pseudo_id))) as conversion_sessions,
count(distinct b.user_pseudo_id) as conversion_users

from first_page_view as a
left join conversion_event as b
on (CONCAT(a.session_id, a.user_pseudo_id)) = (CONCAT(b.session_id, b.user_pseudo_id))

where a.entrances = 1

group by 1,2,3,4,5
order by sessions desc