Here is a good article that help with the use cases for the Lake vs Warehouse decision. There is another Learn article that has a table showing the difference between Datamart, Lake, and Warehouse, but I can't seem to find it.
As for OLTP, I'm not sure that the DW in Fabric would be the best choice. The tables in the DW are just Delta tables, exactly like the lake house. I'm not sure how well a delta table would handle transactional workload. Maybe if it's a highly batched workload it could be an option, but I think a traditional DB would still be the better option for OLTP workloads.
Better together: the lakehouse and warehouse