If copyright holders want to take action, their complaints will go to the ISP subscriber.

So, that would either be the entity operating the public wifi, or yourself (if your mobile data plan is associated with your name).

If you're in a country where downloading copyrighted material can have legal consequences (eg, the USA and many EU countries), in my opinion doing it on public wifi can be rather anti-social: if it's a small business offering you free wifi, you risk causing them actual harm, and if it is a big business with open wifi you could be contributing to them deciding to stop having open wifi in the future.

So, use a VPN, or use wifi provided by a large entity you don't mind causing potential legal hassles for.

Note that if your name is somehow associated with your use of a wifi network, that can come back to haunt you: for example, at big hotels it is common that each customer gets a unique password; in cases like that your copyright-infringing network activity could potentially be linked to you even months or years later.

Note also that for more serious privacy threat models than copyright enforcement, your other network activities on even a completely open network can also be linked to identify you, but for the copyright case you probably don't need to worry about that (currently).

On the public wifi, the operator of that wifi can see any data you pass through their network. They can likely see what sites you visit, but probably can't see what data you send to and from those sites, due to encryption. Unless they have an account with you, or you provide your information in clearext, they can link your data to your devices, but not to you directly, at least not from your use of the AP. They can potentially link your data to your image on their cameras, and thus your identity.

Your ISP has the same access to your data, but they also have a payment account linked to you, and they regularly cooperate with rights holders and law enforcement.

A VPN can do the same thing as an ISP: they know what sites you visit, but probably don't know what data you are sending and receiving, and they can link it to your payment account. However, they generally do not cooperate with rights holders, and may or may not cooperate with law enforcement in their jurisdiction. While you are using a VPN, your ISP knows you are using them, but doesn't know what you are sending back and forth, due to encryption.

If you want to remain as anonymous as possible, use a burner device with no accounts on public wifi.

If you want to avoid harassment by rights holders while you engage in piracy, a VPN is sufficient.

Short answer: Mobile hot spot (w/ your own cellular device) is preferable to public wifi from a security perspective.

There are other considerations, such as how much cellular data downloads cost to you, what sites you’re visiting, what you’re actually doing, etc. In general, it’s advisable to avoid public wifi if you can, but if you must connect to public wifi, then you should make darn sure you connect to the right network (watch out for imposter networks w/ a legitimate looking name) and use VPN (ideally a paid service) to encrypt your traffic. Even with both of these measures, you’re best off avoiding sensitive activities like online banking on public wifi. If you must do banking or other sensitive stuff, either do it on your phone or wait until you get home.

Hope this helps.

Editing to add: When I initially responded, I’d forgotten which community I was in. In this context, I believe the other responses are better than mine, but I’ll keep mine up in case it helps other readers.

Edit: I know, I shouldn't give a shit. But writing a fairly long comment to share my knowledge on this only to see it immediately downvoted without any explanation kind of sucks. So I'm removing this comment and will not interact here anymore.

Use a VPN if you're in the West/Far East. That's it

I have this very same question. Guess I'll just wait for someone more experienced and knowledgeable to enlighten us here in the comments. Sorry if I'm not much help, have a nice day <3