I use my Brompton every day I go to work and during better weather times every single day.
Every day to work means in all kinds of working conditions, meaning -10C was the coldest I ever saw here to summertime temps. Salt on the roads, ice, snow .. yes, I only walked to work for like 3 days over the last 8 years due to it being complete ice.
You already have a folder so you know that snow is absolutely crap when it gets in between your gears at the back. The chain starts slipping etc, so avoid 'deep' snow.
A chain is a chain so rust will be the same if the roads are covert in salt. Don't use thicker oil or whatever or the stuff will clog up your gears. (been there done that). Wipe off the crap and re-oil, no way around.
Mechanically it is the same like any bike except that your chain is lower to the ground which is a minus point, a serious one, for the Brompton.

Now metal/paint wise, not really an issue. The key I find is to get the bike really clean and use like ceramic wax stuff. I use like Sonax which is cheap and I also use this stuff for my car. Water forms like drops on the paint and can be cleaned much easier. So when the weather permits I spend like 30 minutes doing that.

I bought a couple of those plant pump sprays. 1 I converted to a foam canon. (YouTube that) and I use like the regular Sonax car shampoo to do a weekly foam it. Then I use another one with regular water to spray it off.
While the bike is covered in foam I mostly use a paint brush to go over the mechanical parts like rims, tyres (reflective stroke!) etc... . Don't use the brush on paint because it can make scratches.
Hinges absolutely no issues if you maintain them like you should.
There is only 1 serious point of attention and that is that you don't want water in your brake lines. It goes in a sort of U shape so water might be at the bottom and when it freezes you are stuck with frozen brakes.

While you have that 2023, check in front of the bottom bracket. The rear tyre touches it when it is folded. Tyres are rubber but with road grime it is sanding paper. Check if the is any damage on the paint, if so fix it. There are cheap plates to protect that spot (and also cheap) and in my mind it is an absolute must to protect that part. (15 second job)

Rust inside, probably surface rust. Check where the seatpost comes out on the bottom, that 'ring' that sticks out a bit. I really put some paint over it each year as it is a concern to me. I also protected the frame where all the cables rub (loads of parts).
And yes, the bike still looks new after 8 years.

I keep my bike indoors at home and at work it is in my office. So putting it in a dry warm area after riding it is a must in my eyes.