Should I use composite key or primary key from other table

We have a Education Project which has following entities :

• Domain (e.g. Programming, UI/UX, AI, ML), each Domain has 5 Levels (1, 2, 3, 4, 5)

• Building Blocks – which are like small topics e.g. java, multi-threading, loops, prototyping, user interviews. Each Level in the Domain is build of multiple Building Blocks.

• Learning Asset (is like a link to study a concept – it can be associated with multiple Building Blocks) and further these Learning Assets are mapped to a particular Domain -> Level -> Building Block

These are the tables that we have thought of :

Domain

|    id     |      Name          |     
|    10     |      UIUX          |    
|    11     |      Programming   |    
|    12     |      AI            |    

Building Blocks

|    id    |      Name               |     
|    1     |       loops             |    
|    2     |     multi-threading     |    
|    3     |     user-interview      |   

Then we store mapping of Building Blocks to a Domain – Level

Domain-Level-BuildingBlocks Mapping Table

|  DLB_Id    |       domainId   |   level      |   buildingBlockId 
|    100     |        11        |    1         |    1
|    200     |        11        |    2         |    2
|    300     |        10        |    1         |    3

in this table – (domainId, level, buildingBlockId) form a composite key

Learning Asset Table

|    id    |       Name                 |   link     
|    1     |      Loop Notes            |    https://a.com       
|    2     |       Operators            |    https://b.com       
|    3     |       Process and Threads  |    https://c.com 

A Learning Asset can be connected to multiple building blocks

Learning Asset -BuildingBlocks Mapping Table

|    id    |       learningAssetId  |     buildingBlockId 
|    1     |        1               |        1
|    2     |        2               |        1
|    3     |        3               |        2

Now the Admin can select if a Learning Asset is applicable in Domain-Level-BuildingBlock combination so

Learning Asset-Domain-Level-BuildingBlocks Mapping Table (Table A)

|    id    |       learningAssetId  |   domainId   |   level      |   buildingBlockId 
|    1     |        1               |      11      |    1         |      1
|    2     |        2               |      11      |    1         |      1

in this table – (domainId, level, buildingBlockId) form a composite key

My question was in the above table : should I again store (domainId, level, buildingBlockId) or should I use their primary key DLB_id from the Domain-Level-BuildingBlocks table

something like this :

Learning Asset-Domain-Level-BuildingBlocks Mapping Table (Table B)

|    id    |       learningAssetId  |   DLB_Id   
|    1     |        1               |      100   
|    2     |        3               |      200     

1. My question is whether to use Table A or Table B

2. If Table B is the correct way should I generate DLB_Id as string by combining domainId + "-" + level + "-" + buildingBlockId instead of using Auto-increment Integet Primary Keys. Will the indexing on the generated string be as efficient as the Auto-increment Integer Primary Key. The reason for generating the string is that when we need to fetch the Learning Assets that belong to a Domain-Level-BuildingBlock combination I don't need to use the Domain-Level-BuildingBlocks Mapping Table – rather I can directly look up the generated string Id in the Learning Asset-Domain-Level-BuildingBlocks Mapping Table

Learning Asset-Domain-Level-BuildingBlocks Mapping Table (Table C)

|    id    |       learningAssetId  |         DLB_Id   (as generated string)
|    1     |        1               |        '11-1-1'
|    2     |        3               |        '11-2-2'

i.e. Table B or Table C ?

Thank you