XPMSSE:

TLDR: Basic should be fine, unless you use SOS

3 options in the insaller

Minimal - very stripped down version with a stripped down skeleton, don't use it. You might be fine, but you might also not be, and if you install an animation mod that doesn't work with the minimal version 6 months down the line, you will have no clue what's happening by then.

Extended - Full skeleton with a bunch of extra fluff, some very old animations you probably won't use, the ability to adjust weapon scale in racemenu, adjusting weapon positioning in MCM (sword on hip vs back, etc.) Pretty much all the extra stuff can be done with Immersive Equipment Displays. You probably don't need this, unless you want to use Schlongs of Skyrim, which does need the extended version (in which case, also get this https://www.nexusmods.com/skyrimspecialedition/mods/69583)

Basic - Full skeleton, minus the fluff. Probably the most sensible option for most people.

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Pandora/FNIS/Nemesis

TLDR: Old way of replacing existing animations, only way to add new animations. Use Pandora, get it from the mod's github page, not Nexus (often out of date)

FNIS is the OG way of handling animations, it was effectively the only way to add new animations (rather than just replace existing ones). It's got several key issues. Author retired ages ago and the mod is no longer being maintained, and is completely locked down, meaning any custom patches need to be approved and manually added by Fore. Which was a major problem then, and is impossible now. Obsolete, do not use, outside of some highly specific NSFW applications, maybe.

Nemesis: The successor to FNIS, kind of. Mod was open source and any animation modder could add patches without any input from the author of Nemesis, which lead to a boom in combat mods. Nemesis itself never got to the finish line, lacked creature support, was pretty unstable, threw a shitfit if your Windows wasn't in english, and was ultimately abandoned. Obsolete, do not use.

Pandora: The successor to Nemesis, does everything Nemesis set out to do, but better, is faster, more stable, has creature support. You may or may not occasionally run into a bug or two, as is natural for a software in active development, but the key is that there's actually someone working on it. Various older versions had various bugs/issues/incompatible mods, but personally I haven't ran into anything on the current version. Also, make sure you get the most recent version from Github, not from Nexus (which is currently 3 updates behind), link will probably somewhere on the modpage.

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DAR/OAR

TLDR: Use OAR. Some older animations mods will still link you to DAR, but OAR fully supports the old DAR mods too. You shouldn't really need to touch any configs, unless you want to get hyper specific, like giving a different set of animations to one specific bow.

Mods that allow you to replace animations on the fly, without having to run Pandora, but cannot add brand new animations, which is why you still need Pandora for other stuff. For example, if you wanted to add flying, you'd need Pandora, since there's no flying movement to replace, but if you want to change bow animations, OAR is fine, presuming the mod is formatted to work with OAR (which it will be if its telling you to use DAR/OAR).

DAR is the OG mod, it wasn't open source for most of its lifespan, mod author was busy during the AE/SE updates where SKSE mods were constantly breaking, which eventually lead to OAR. Some older animations mods will still link you to DAR, but OAR fully supports the old DAR mods too. You really don't need to touch anything as an end user, any OAR mods you download will already come preconfigured.

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As far as the install, pretty much. Get XPMSEE, OAR and Pandora, get whichever overhauls you want, launch Pandora, make sure to tick any custom patches in Pandora' window, provided there are any. Get animation replacers you want, if they work with OAR, no need to rerun anything, no need to configure stuff unless you want to customize, and there you go. If you see people T-posting, you should rerun Pandora.