As you'll see from the various contributions to this thread you won't routinely be able to obtain Paxlovid in Europe because most cases simply don't require it, so you won't be drawing anyone's attention to yourself.
Any requirement to quarantine is down to country-specific laws and your personal moral compass in terms of the risk of infecting others. I'm not aware of any country in Europe that mandates quarantine. Given that almost no-one is now testing, so there will be thousands of people out-and-about blissfully unaware they have Covid, any different approach would be something of a nonsense anyway.