Cleaning up with sed 2

I have a list of nucleotide sequences in fasta format, obtained from NCBI. I want to clean the sequences names, leaving only the accession numbers. Additionally, I want to extract the accession numbers to a separate list. I am not that familiar with sed, since I use it mainly in my SlackBuilds to fix small things. Therefore, for my fuuture reference, I decided to sum things up.
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