It's great, works perfect / exactly as wanted. Many thanks Rahul, you got the logic.

So  I understand that :

• Above all, I create a new set , where i'll store the string that I want to compare (as many sets as there are fields to be compared)
• Then I need tomake a 1st 'for' Loop, where I'll store values in my maps
• And finally Loop through all leads processed by the trigger and insert my keys.

->True ?

I made some little changes to the code so as to make it ready to use :

trigger AutoAssignDupesOnLeadInsert on Lead (before insert) {
    Set<String> setLeadEmail = new Set<String>();
    Set<String> setLeadCompany = new Set<String>();
    for ( Lead objLead : Trigger.new){  
        //fetch the Email, needed for SOQL
        setLeadEmail.add(objLead.Email);
        //fetch the Company, needed for SOQL
        setLeadCompany.add(objLead.Company);
    }

    Map<String,Account> mapAccount = new Map<String, Account>();
    //Create a map of Name, Account
    for(Account a : [SELECT Name, OwnerId FROM Account WHERE Name=:setLeadCompany AND IsDeleted = false ORDER BY CreatedDate DESC]){
        mapAccount.put(a.Name, a);
    
    }
    //Create a map of Email, Contact
    Map<String, Contact> mapContact = new Map<String, Contact>();
    for(Contact c : [SELECT email,OwnerId FROM Contact WHERE email=:setLeadEmail AND IsDeleted = false ORDER BY CreatedDate DESC]){
        mapContact.put(c.email, c);
    }
    //Create a map of Email, Lead
    Map<String, Lead> mapLead = new Map<String, Lead>();
    for(Lead l : [SELECT email, OwnerId FROM Lead WHERE email=:setLeadEmail AND IsDeleted = false ORDER BY CreatedDate DESC]){
        mapLead.put(l.email, l);
    }

    for(Lead l : Trigger.new){
        //Check if Company is present in map
        if(mapAccount.containsKey(l.Company)){
            l.OwnerId = mapAccount.get(l.Company).OwnerId;
            l.status = 'Duplicate Account Found';
        }
        //Check if Email is present in map
        else if(mapContact.containsKey(l.Email)){
            l.OwnerId = mapContact.get(l.Email).OwnerId;
            l.status = 'Duplicate Contact Found';
        }
        //Check if Email is present in map
        else if(mapLead.containsKey(l.Email)){
            l.OwnerId = mapLead.get(l.Email).OwnerId;
            l.status = 'Duplicate Lead Found';
        }
    }
}