I can help answer at least a couple of those questions:
Does GPT-3 Semantic Search work for other languages too?
The vast majority of the training set is English, but there was enough content from other languages to give GPT-3 some modest multilingual capabilities. Performance on English text tends to be significantly better than other languages.
Briefly, how does GPT-3 do semantic search under the hood?
The search query and each document are combined into prompts that are sent to the model. The computed log probabilities of the tokens in the document are used to produce a score indicating how likely it is that the document is related to the query.