There's a mix, a quick search took me to https://londonbusadvertising.com/ which shows wraps, which aren't going to be replaced with a display. But also rectangle panels which could be replaced with displays.
Those panels might very well be vinyl for outdoor durability, but I don't see why they'd take 3 days to swap out, unless it's a scheduling/transport issue, for example a bus operator needs to drop off day before, so the ad company doesn't have to schedule around when the drop off happens, and the bus operator picks up the bus the day after, because they don't want to schedule around when the ad company finishes; now your one hour swap is a multi-day production.
I'd love this to be banned. Not only is it a visual eyesore form the street, it devalues public transport's brand, and in many cases it makes it hard for people inside the vehicle to see where they are.
I love our buses in Kraków, Poland. They mostly don't carry any advertising on the outside, but when they do, it's advertising the fact that the bus is fully electric, zero-emissions, and part of the new all-electric fleet. It's low-key, aesthetic, and basically advertising public money being well-spent on improving QoL for citizens.
(I may be wired weird; I'm also happy when I see signs on stuff saying it's been financed by Local Program X, Subprogram Y, with support from EU Program A, Subprogram B, Function C, blahblah. Unfortunately not everyone cares to make those look aesthetically, given that the information is only placed because it's a condition of the grant, but it usually looks OK and IMHO sends a positive message.)
EDIT:
Trams here have been seen carrying exterior ads for private businesses every now and then, less so now than in the past; these days, it's mostly either default coloration or some temporary "this train is new and awesome" ad.
Bus stops, however, are another matter.
As for internal screens, sometimes ads find their way onto the "bus TV" and "tram TV" displays. Most of the time, it's a mix of tourist trivia, air quality report, PSAs (safety warnings, transit etiquette), and transit org's own ads (showing off new eco-friendly fleet, job ads). There's a separate set of screens that show a map (OSM!) and the route with upcoming stop markers, but unfortunately, half the time they're broken - either the map or route indicator is frozen, or they get desynced from each other, or reality. Voice announcements seem to be a separate system and are usually reliable, though every now and then they desync from reality too.
I sometimes wonder who's maintaining this and if they'll take a volunteer (or part-time contractor) to help them keep the indicators working.
> I may be wired weird; I'm also happy when I see signs on stuff saying it's been financed by Local Program X, Subprogram Y, with support from EU Program A, Subprogram B, Function C, blahblah.
Do they also carry the name of the local politician who runs the program? That should raise some eyebrows...
> That sounds like wrapping a whole bus with an ad.
They were talking about the standard landscape side panels. Didn't make much sense to me either but that was why the bus companies were throwing money at them to get LED panels working (aside from the financial bonus of being able to book multiple ads for the same bus, obvs.)
(As an example of how efficient TFL's advertising swapping was - there was a poster at Deptford Bridge DLR advertising a Gorky exhibition in 2010 that wasn't changed until late 2017/early 2018. And all that involved was opening the street-level case to put in a new poster!)
Those panels might very well be vinyl for outdoor durability, but I don't see why they'd take 3 days to swap out, unless it's a scheduling/transport issue, for example a bus operator needs to drop off day before, so the ad company doesn't have to schedule around when the drop off happens, and the bus operator picks up the bus the day after, because they don't want to schedule around when the ad company finishes; now your one hour swap is a multi-day production.
A full wrap, could be a 3 day process though.