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 https://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, https://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 is built with different video extraction technology under the hood, based off the very popular NewPipe app and developed by the same team. Here is a list of public piped instances. piped.video is the most popular instance and you have probably seen a lemmy bot in the comments provide a piped alternative link whenever someone links a youtube url. Download is also hit or miss youll have to try a few piped instances 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.

If you want to be sold a little bit more on using invidious or piped, here is louis rossmann talking about why he ditched yt premium for it and why he recommends his viewers do the same regardless of his 'loss' in ad revenue