To say BTC has a fixed supply is kind of wrong. It will increase until the year 2140 and then it is capped and won't grow any longer.
And it is also wrong to say Monero's supply is infinite because for everyone currently the supply is steadily inflating about 0.85% per year, just like the BTC supply is. They're currently basically identical.
Only if you look at them at a infinite timescale, then you could argue that Monero's supply is infinite, but no human could experience that, so it basically is a useless metric.
