It's shocking to me how many riders still have no idea they need to wear ear plugs. Yes, even behind a fairing and in a quality helmet, you still need ear plugs.
Yes, you can still hear the things you need to hear with ear plugs in. Much better in fact. There's no such thing as a magic "cone of silence" that eliminates all sound. Ear plugs only reduce sound so it's below damaging levels, and so you can actually hear things like sirens and horns far better than if your head is full of wind roar.
Every ear is different, so every rider has to experiment to find the best solution that works for them. You can buy sampler packs online of various styles of disposable ear plugs if you'd like to try a variety. There's a way to plug any ear.
I'll share my personal anecdote that may be useful. Your ear plug journey will likely be different.
Anyway, the custom ear plugs have never worked for me. They don't work for a large percentage of folks; I think perhaps some people have ear anatomy that moves around more or something. Things are nice and quiet, then the helmet goes on and the plugs lose their seal. And even at their best, the NRR of custom ear plugs is far less than good foam plugs.
Custom plugs (including plugs with ear phones) do work for many, but you need to be aware going in that they don't work for some. There are no guarantees. You'll always get better noise reduction and protection from foam plugs.
Every ear is different, and some folks even find that they need to use something different in their left ear than the right.
After two EXTREMELY painful ear canal infections caused by re-using ear plugs, I now only use disposable foam ear plugs and replace them daily. There are several models that worked well for me (Howard Leight was what I used to use most of the time, and they work well for lots of people).
However, I lost a large amount of weight and found that my ear holes got bigger and my usual ear plugs wouldn't seal any more. After some furious Googling, I found that the largest foam ear plugs in all the lands were "Hearos Xtreme", which is what I use now. I buy them by the box of 200 on Amazon.
If you have narrow ear canals, there are also smaller ear plugs available made for women and children. Some folks with very sensitive ears, hard-to-fit ears, or non-standard ear anatomy have to use silicone putty ear plugs, where you mash a little blob into your ears.
I'll also note that one of the most common ear plugs provided by workplaces (the EAR Classic, a yellow foam cylinder) tends to be very abrasive and uncomfortable with much harder foam than most, and has given many people a negative impression of foam ear plugs. There are far more comfortable options.
The other thing to remember is that there's a learning curve to this all. Of course, if you have any pain or dizziness, you need to remove or reposition your ear plug immediately. But for many people, it takes a little time to get used to using ear plugs at first. After a while, the ear plugs should "disappear", but they do feel odd at first. I now often use ear plugs to sleep if things are noisy (snoring roomie at a rally, for example).
With any ear plugs, there's an art to inserting them correctly; you need to roll up foam ear plugs evenly and insert them at the right depth so they're effective and comfortable. Give yourself some time to learn all this, and experiment with different ear plugs and techniques.
It's well worth it; you'll be able to ride further, faster, and far more safely because you'll massively reduce fatigue and distractions. And of course, you'll be able to hear your grandchildren...