Hello, I am new to cryptocurrency development, and I have noticed that Bitcoin has an official wallet for both Desktop and Mobile (correct me if I am wrong).
There's no official client. You might be referring to Bitcoin Core which is considered the reference implementation with it's protocol rules in line with the consensus. There isn't any official mobile wallet as well.
My question is, does Bitcoin have an official explorer and value API (such as Bitcoin to fiat conversion) too? If not, are there services or APIs for developers that are recommended and widely used?
No. I'd say Blockchair has a nice API with the data dumps but for the conversion, it will vary across the various exchanges and you'll be better off getting the information from the exchange's API.