Dear @LeileifromUK , you would not have to resurrect this post from the dead, as @mlterryMitre had a similar question not long ago.
You are indeed very right, an update is dearly needed. Please read through that other post and if you have more questions, please post them there.
What Anna explained 3 years ago was right when only SNOMED US was in scope and we did not do a lot about mapping into SNOMED for a refresh. Now we collect 3 different versions, merge them and try to fix the mappings in order to provide excellent working conditions for the OHDSI community. With all these dependencies the complexity of the process has exponentially grown.