• Command substitutions and variables: Fish for bash users — fish-shell 3.7.1 documentation
• string collect: string-collect - join strings into one — fish-shell 3.7.1 documentation
• SO: Read file into environment variable? · Issue #7323 · fish-shell/fish-shell:

set VARNAME (cat ~/myfile | string collect)

Here string collect makes sure it’s a multiline variable instead of an array composed of one element per line.