There are many websites that exist that act as web front ends for youtube-dl/dlp you may want to give consideration to such as youtubemp3free.com/en/ Unfortunately these tend to be very slow to convert as they have a lot of people using them all the time.

Many invidious instances/websites offer video+audio format download functionality. If you don’t know what invidious is its a free and open source front end/scrapper for youtube that usually offers a better user experience than youtube premium. Here is a list of all public instances, vid.puffyan.us is my go-to but is currently having rate-limiting issues for being too popular so the vid download function is broken. However I managed to get another instance, invidious.slipfox.xyz/feed/popular to download a vids audio in .mp4 format.

There is also piped, similar to invidious in spirit and function but built off the NewPipe apps technology. Here is a list of public piped instances. Download is also hit or miss youll have to try a few yourself to find one that works probably.

Unfortunately your options are rather limited because IOS is so locked down. If you had said android instead I could offer you more and much better options but well it is what it is. I hope that you at least try out some invidious or piped instances and find one you really like.