Read Committed Shared Lock I wanted to know when you are at read committed isolation level and in one session someone started an explicit transaction which has a select statement, say it takes 20 seconds. Can someone from a different session will be able to update the records from that table while the select statement is still executing.
I was discussing this with someone and according to them it will be able to update those records because that's where update locks come into play and gives the exclusive lock to the update while the query is still executing.
I can't find any documentation on this and also don't know how to test this
I can't find any documentation on this

Docs are here.  An S lock is compatible with a U lock, but the UPDATE will reuquire an X lock to actually complete, which is incompatible with an S lock.


and also don't know how to test this

You can always run a a query like
select count(*)
from sales.SalesOrderDetail with (tablock), master..spt_values v, master..spt_values v2, master..spt_values v3

And see if you can update from another session.  The tablock is to ensure that the whole table is locked with an S lock, as it may or may not be with a long-running SELECT.
You'll discover that unless the database has the READ COMMITTED SNAPSHOT option set, the SELECT will block the UPDATE.