[SalesForce] Inner query in soql can only happen for master-detail relationship or also for lookup relationship

If you are passing hardcoded id ( Which you should not) then your query will look like

[select id, Description from expenseItem__c where expense__c = 'parentExpenseId' order by createdDate DESC];

While if you have Id in some variable then your query will look like

Id parentRecordId = 'SomeValueHere';
[select id, Description from expenseItem__c where expense__c =: parentRecordId  order by createdDate DESC];

select Id, FamilyMember__c
from A
where StartDate__c > TODAY
    and EndDate__c < TODAY
    and FamilyMember__c in (
        select Id
        from B
        where Membre__c = true
        )

this SOQL is split into 2 parts:

• select Ids of B with condition (Membre__c = True), this is inner select
• select A with condition (FamilyMember__c is in ids of records from previous step)