Populating fact table with key's from other tables

Question

I hope you guys could help me with solving the following problem.
I am new to SQL (using SQL Server Management Studio 2017).

My goals is to combine the keys of different tables into one table.
Which is shown below. For this example i have 4 source tables of which i only display the keys;

ContractTable:

+------------+---------+
| ContractId | PartyId |
+------------+---------+
| C01        | 1       |
+------------+---------+
| C02        | 2       |
+------------+---------+
| C03        | 3       |
+------------+---------+
| C04        | 4       |
+------------+---------+
| C05        | NULL    |
+------------+---------+

<!-- end snippet -->

PartyTable:

+---------+
| PartyId |
+---------+
| P1      |
+---------+
| P2      |
+---------+
| P3      |
+---------+
| P4      |
+---------+
| P5      |
+---------+
| P6      |
+---------+
| P7      |
+---------+
| P8      |
+---------+

RentalObjectTable:

+----------------+------------+----------+
| RentalObjectId | ContractId | ObjectId |
+----------------+------------+----------+
| R1             | C5         | O1       |
+----------------+------------+----------+
| R2             | NULL       | O2       |
+----------------+------------+----------+
| R3             | C4         | O3       |
+----------------+------------+----------+
| R4             | C1         | O4       |
+----------------+------------+----------+

ObjectTable:

+----------+
| ObjectId |
+----------+
| O1       |
+----------+
| O2       |
+----------+
| O3       |
+----------+
| O4       |
+----------+
| O5       |
+----------+
| O6       |
+----------+

Together they should form the Key/FactTable

+---------+------------+----------------+----------+
| PartyId | ContractId | RentalObjectId | ObjectId |
+---------+------------+----------------+----------+
| P1      | C01        | R4             | O4       |
+---------+------------+----------------+----------+
| P2      | C02        | NULL           | NULL     |
+---------+------------+----------------+----------+
| P3      | C03        | NULL           | NULL     |
+---------+------------+----------------+----------+
| P4      | C04        | R3             | O3       |
+---------+------------+----------------+----------+
| P5      | NULL       | NULL           | NULL     |
+---------+------------+----------------+----------+
| P6      | NULL       | NULL           | NULL     |
+---------+------------+----------------+----------+
| P7      | NULL       | NULL           | NULL     |
+---------+------------+----------------+----------+
| P8      | NULL       | NULL           | NULL     |
+---------+------------+----------------+----------+
| NULL    | C5         | R1             | O1       |
+---------+------------+----------------+----------+
| NULL    | NULL       | R2             | O2       |
+---------+------------+----------------+----------+
| NULL    | NULL       | NULL           | O5       |
+---------+------------+----------------+----------+
| NULL    | NULL       | NULL           | O6       |
+---------+------------+----------------+----------+

Uptill now i tried the following:

CREATE TABLE FACTS(
ObjectId NVARCHAR(255),
RentalObjectId NVARCHAR(255),
ContractId NVARCHAR(255),
PartyId NVARCHAR(255)
);

INSERT INTO FACTS(ContractId)
SELECT ContractId
FROM ContractTable

UDATE FACTS
SET FACTS.Party_id = PartyTable.PartyId
FROM FACTS FULL OUTER JOIN
PartyTable on PartyTable.PartyId = FACTS.PartyId;

Using that same update statement for the other field.
However this does not work.
I do not get the values on which the query does not find a match.