If you know more JavaScript, then following improvements would be great idea:
By default
show the full table with all currencies as you did originally (since that's nice to see as a first thing)
set initial "units" value to be 1 automatically.
let user input the "Units" and as soon as user inputs the number it could recompute the whole table automatically - that would be cool and visual.
then if user wants to select specific currency, then yes, maybe filter down just to that single row.
Also
It would be good to do smart formatting of the numbers, e.g. instead of 21962.86467679295 sats you could do 21'962.9 sats (since more decimal places don't make sense anyway)
it would be also good idea to have a solid scheme in the URL, e.g. sats2fiat.html?u=123&f=eur (so the specific value can be shared with someone)
21962.86467679295sats you could do21'962.9sats (since more decimal places don't make sense anyway)