I googled for
Dark Angels heraldry,
Dark Angels insignia, and
Space Marines heraldry to get an overview of what type of guides are out there (it's also easier to show in pictures than to explain it all in text, plus if it's out there then I might as well link to it). As far as I remember (maybe things have changed over the years, I've not kept up to date). Dark Angels are still somewhat Codex compliant when it comes to insignia, meaning that if something is not explained as the Dark Angels way of doing things then they use the regular Space Marine way of handling that type of heraldry.
Here are a few guides (I looked through them and they all seem accurate to what I remember).
First of all the generic Space Marine way of doing things. Understanding that should give you a baseline for what types of icons can show up, why they are used, and where they tend to show up:
https://wh40k.lexicanum.com/wiki/Codex_Astartes#Heraldry and this thread has a few more images about generic Space Marine heraldry (even as the thread starts out being about Canadians/Dark Angels) if you scroll down a bit:
https://www.reddit.com/r/Warhammer40k/comments/14ur5b3/whats_this_canadian_looking_chapter_on_the/
Then a bunch of honour badges that can be applied to make your Marines a bit more unique:
https://wh40k.lexicanum.com/wiki/Space_Marine_Honour_Badges
A thread with some more details about Dark Angels heraldry:
https://bolterandchainsword.com/topic/366733-dark-angelsknightly-heraldry-guides/ and they got a page with additional (generic) resources:
https://bolterandchainsword.com/topic/282443-dark-angels-resources/
This page has a few points about squad and company markings:
https://www.goonhammer.com/how-to-paint-everything-dark-angels/
Another page with details of how their heraldry works:
https://david.ely.fm/2021/03/03/shoulder-pads-of.html
And another one:
https://wargameexplorer.com/post/space-marine-heraldry-and-armor-in-warhammer-40k/
Looking at the transfer sheet (from here:
https://i.ebayimg.com/images/g/TSkAAOSwsqJklhxV/s-l1600.jpg ) it looks like the Dark Angels row (the second one) got (from left to right) the white chapter icon for power armoured troops and its leaders, then the red chapter icons (Deathwing/Terminators have "broken winged swords" as their icon) plus a bunch of honour badges (the small white ones), then the "explosions" as squad designators (on the shoulder) and Crux Terminatus (cross with a skull in it, as squad designator for veterans), and finally a few bigger markings for vehicles.
When it comes to numerals then Dark Angels generally use the regular ones, not the Roman numerals. At the bottom left you got those in black and white. Some of the guides linked probably show which colour is used for what but my guess would be that the black ones are for white/Deathwing/Apothecary miniatures and the white ones are for dark green miniatures and for squad numbers over the red squad designator icons. That's also why there are the big 2, 4, and 8 in white and 1, 7, and 9 in black, and a single yellow 5 (because Blood Angels also use those numbers) for vehicles (you can probably paint your own numbers, those are simply the ones they provide on the transfer sheet). That way you can use them for Deathwing or Ravenwing (black)/regular Dark Angels (dark green).
And finally below the Roman numerals you have a bunch of honour badges (see guide linked above for what they can mean). The first seem to be a bunch of elongated Crux Terminatuses, then a few Imperial Eagles (Aquila), some skulls, then the second row starts with two Iron Halo (red circle with spikes), more eagles, and something I don't recognise at the end.
Here's also a site that has generic transfer sheets if you need more (haven't bought there, don't know the quality but the creator also comments here on dakka, if I remember correctly). They are not all 100% Dark Angels decals but many are more than similar enough and available in multiple colours:
https://www.themightybrush.com/product/skulls-v2-waterslide-transfers-decals/ (because skulls work for any Space Marines)
https://www.themightybrush.com/product/skulls-waterslide-transfers-decals/ (the same in white)
https://www.themightybrush.com/product/halo-skulls-transfers-decals/ Iron Cross + skull
https://www.themightybrush.com/product/retributor-markings-waterslide-transfers-decals/ (some looks Dark Angels-ish)
https://www.themightybrush.com/product/check-pattern-transfers-decals/ (they once used to have quite a few chequered decorations, like on the leg or at the bottom of the shoulder pad (just above the rim), or on their vehicles)
https://www.themightybrush.com/product/imperial-gothic-numerals-waterslide-transfers-decals/ Numerals fitting for Dark Angels
https://www.themightybrush.com/product/double-headed-arrows-transfers-decals/ tactical squad marking
https://www.themightybrush.com/product/explosions-transfers-decals/ devastator squad markings
https://www.themightybrush.com/product/fast-attack-arrows-waterslide-transfers-decals/ assault squad markings
https://www.themightybrush.com/product/crossed-swords-transfers-decals/ useful as a Dark Angels honour badge
You can also paint the iconography/heraldry onto your miniatures.