The following document provides links to helpful resources relating to FASTA files from the UniProt knowledge base.
https://www.uniprot.org/downloads. Here is the downloads page for the UniProt knowledge base. Click on the FTP link to find the specific FASTA file of interest.
https://en.wikipedia.org/wiki/FASTA_format. Here is general information on the FASTA format.
https://blast.ncbi.nlm.nih.gov/Blast.cgi?CMD=Web&PAGE_TYPE=BlastDocs&DOC_TYPE=BlastHelp. This link to the NCBI provides more information on the FASTA file format
https://www.uniprot.org/help/fasta-headers. This page from UniProt explains how to decode their FASTA headers (i.e. what does the SV number or PE number mean).