Some jerseys are composed largely of mesh or other thin fabrics largely transparent to UV. As someone who never tans and always burns, wearing clothing to ward off UV is often at least as comfortable as slathering oneself in sunscreen; YMMV.
I agree though that more layers are hotter, especially when it’s humid; if there’s any fabric that’s actually comfortable in high humidity - as opposed to less uncomfortable - I’ve never found it.
I wear the Craft sleeveless mesh base layer in the warm and hot months. I’ve noted that though I am slightly warmer wearing one, I am much more comfortable. Plenty of humidity and days in the 90s where I live. Love this question! Thanks!
I wear a sleeveless mesh type base layer in the summer. I’m not 100pc sure it’s cooler but I do find it more comfortable. Also on a cooler start with arm warmers it seems to help.
If I recall the article I read about this, it is possible for helmets to be cooler because, although no vents will give as much airflow as no helmet, a helmet also blocks the sun and foam is very bad at transferring heat. It, of course, depends on the details of the test protocol.
I have no idea about the jacket.
I always have a base layer on because mostly I hate the feel of bib straps against my skin. Also I don’t want my bra or my chest exposed.
Always base layers for me as well. Once it’s really hot I find the wind passing through the damp base layer helps a bit with cooling when I open my jersey.
I also think it’s just a bit nicer to be covered up. For myself and others.
I wear a Brynje wool mesh base layer year-round. Sleeveless, short sleeve or long sleeve.
In summer it can be 105F and I usually use the sleeveless micro mesh under an ultra lightweight merino jersey. When I partly unzip the jersey the mesh lets the breeze move around my body.
https://www.brynjeusa.com/product-category/base-layers/wool-thermo-light
I try but just can’t get cool enough with a base layer. Even the zero-weight one feels restrictive on hot days, particularly with humidity. I run hot and they don’t help. 
I feel like those arguments were in defence of the safety items. Like, helmets are too hot. Ok but for the safety they provide they’re pretty close to no helmet. Which I understand but it’s not entirely truthful.
Dunno how humidity is measured where you live, but 100% is the max round here. Which is unpleasant in the extreme.
If it was a thing, you’d see the pros wearing them. I don’t think I’ve ever seen a single pro wear a base layer climbing up in the alps.
I still wear it though, my jerseys fit better and the straps minimise chaffing.
If you ignore any information produced by anyone that sells baselayers, and just think about it logically, it is obvious.
You get out the shower, you are wet, you feel cold. You dry yourself, you are not cold.
If you want to be cool your skin needs to be wet - no baselayer.
If you wan to be warm your skin needs to be dry - baselayer.
If we didn’t need to be wet to cool down when we are hot, we would not have evolved to sweat.
I use mesh base layers and find that they keep me cooler when the temps hit 25C+ (77F). The Decathlon ones are sound. I have tried Temu but the quality isn’t the same, especially with repeated washing
I think it’s a personal thing and depends a lot on the base itself. For some conditions I’ll go with a Defeet base all the way up to 75. Otherwise I skip it altogether. For humid days, I’ve occasionally used an old cotton t-shirt that I cut holes into. I’ve had it for years, originally used with wool jerseys. Why? Cotton tends to stay damp when wet, so when it’s hot and humid that damp t-shift acts as a cooling layer.
Yes a super light one by Rapha but mostly because I hate the feeling of straps on my body than for cooling or drying. Also nobody wants to see my non-dad dad bod if I want to open it.
And btw it maybe a bit warm on the climb but it can help while descending (providing it’s one that’s fast drying)
y
Y’all just ain’t trying hard enough.
I like them on long rides to prevent chafing.
I feel they’re a bit warmer on the bike but once I stop, they feel quite a bit warmer.
I like them in mixed/alpine condition because they seem to keep the moisture of my body on descents so they help with temp regulation as I unzip going up and feel less wet going down.
Always as I personally dislike the feel of the wet jersey sticking to my skin. No issues whatsoever in the heat and humidity either.
I wear them most of the time for similar reasons already stated and they keep the sweat out of my shorts. Dryer is better.
Tom, I have tried some of the mesh base layers and feel they make me warmer than running without a baselayer when it’s summer. I wear a baselayer most of the other 355 days of the year when it isn’t summer in Scotland. If I head abroad to Europe, I’ll take some baselayers incase the temps drop but don’t need them too often.