What do I need to wear visiting the Swiss Alps in summer?

Layering is the answer. There are lots of posts in the packing section that explain the concept if you are unfamiliar.
Here's a post that may help.
https://community.ricksteves.com/travel-forum/packing/packing-suggestions-for-my-way-alpine-tour-in-august-for-a-woman

The mornings and evenings are chilly in the Alps. My packable down jacket worked well, but I also needed my packable full- length raincoat. A rain poncho would also be functional. I also applied a waterproof spray to my favorite walking shoes. Better safe than sorry!
Have a great trip in this beautiful country.

I leave in 4 days and am having the hardest time deciding what to bring! I am super temperature-sensitive, i.e. I am literally melting when over 80°, But, "freezing" below 60°. I'm stuck on how many shorts vs. Pants to bring. Tops are easy b/c I just bring all tank tops and then a fleece jacket and waterproof jacket as needed and I'll wear a sweatshirt on the plane. But it's hard trying to gauge what to do for my lower half, lol. If I could bring a larger suitcase I'd be all set but I'm trying to keep it to carry-on size. 🥴

My favorite sign up on the top of the Schilthorn. Will save some packing spce and then you also wonder…who would??

https://isaiahcastillo.wordpress.com/2022/08/10/a-funny-sign-on-the-trail-to-schilthorn-dont-go-hiking-on-stiletto-heels-in-the-swiss-alps/

Normally the last few summers it has been warmer rather than colder. Yes, it can be cool in the morning/evening and it can be cold if you go up to peaks like Jungfraujoch or Gornergrat, but I wouldn't overdo it with cold weather gear in July or August. A decent fleece and/or packable down-style jacket, and you could throw in a neck gaiter/gloves/hat that won't take up much space as backup if you want. If you have silk or other thin long underwear, that also makes a good extra layer, lightweight, easily transported and then can be worn as/if needed. Breathable rain gear is a must - there are often thunderstorms late in the day or showers in the early morning. I have never brought a winter jacket to the mountains in summer - max a fleece layered under a rain jacket, usually ending up with me taking both off as soon as the sun comes out. Hiking pants are great - very thin and light for packing, work for both warm or cooler weather (especially if you get the kind that can be converted into shorts or capri-length).
In my experience, Berner Oberland also tends to be a bit warmer than other mountain areas like Graubunden or Zermatt. And the sun is very strong up in the mountains, so I also recommend a sun hat of some kind. I always hated them and resisted for a while, but eventually realized it is just a necessity when hiking or walking in summer here.
And fyi for anyone coming this way in the next week or so - this weekend through Monday is going to be HOT (90+) in the cities and getting more humid...even Wengen will be hitting 80.

I was in Wengen for 5 nights last week. Temps were in the 70s with one day of rain. We were wearing shirts most days. We went to Jungfraujoch on a clear morning (arrived just after 9) and there was no wind. I wore long pants, a t shirt, a packable puffy jacket and a beanie hat. I could have taken my jacket off outside except the sun was very strong and I would have been burned quickly. We saw several people badly burned over the time in the area. Sea glasses are critical. The coldest area was in the ice cave inside the mountain.

A rain jacket or poncho is critical. We started the day in clear skies in Wengen, and got caught in a rainstorm on First above Grindelwald. Of course we didn’t have our rain gear with us!

And the sun is very strong up in the mountains, so I also recommend a
sun hat of some kind. I always hated them and resisted for a while,
but eventually realized it is just a necessity when hiking or walking
in summer here.

Listen to hopper on this one. I got so sunburned last year and am not allowed to return until I get myself a hat. 😊

I just got back from 12 day tour in Switzerland (RS) and it was HOT! we def packed wrong. Bring shorts, t-shirts and just a light sweatshirt maybe a few pairs of comfy pants.. thats it! you can always buy stuff there (plenty of places and beautiful clothes, all price points..especially in Luzern or Lausanne , Lugano , Bern..they have dept stores that are great(cheaper than boutiques)..

We are in the hills above Vevey/Lake Geneva now. It's 11 AM, lovely, with a small breeze. We are in shorts & flip-flops.

A puffy vest is always useful and stylish :)

I'm usually always warm, and was wearing t-shirts with leggings in the mountains (sometimes the leggings were capri length) and was fine except when over in Murren/Gimmelwald and it was raining. I used a light rain jacket and was totally fine. I don't think I really used my fleece except one day of that whole week. We were in Grindelwald, Murren, Gimmelwald and Lauterbrunnen two weeks ago.

"My favorite sign up on the top of the Schilthorn. Will save some packing spce and then you also wonder…who would??
https://isaiahcastillo.wordpress.com/2022/08/10/a-funny-sign-on-the-trail-to-schilthorn-dont-go-hiking-on-stiletto-heels-in-the-swiss-alps/"

When we the agent showed us around the house we ended up buying in Wengen he told us that before us he had shown it to a couple where one was wearing high heels. They didn't buy :-)
High heels are not really useful here.

Get practical stuff. I use trailrunners as my every day shoes. Have layers. Always carry a rain jacket when hiking in the mountains. Bring one pair of thermal underwear. They do not take a lot of space, and can turn any outfit it to a winter one.