so can i make a derivative asset that uses mixed assets? 

Yes. Unless you want the derivative under a single license, in which case "probably".

First, let's cover that "Yes": Derivatives can be placed under multiple licenses of their sources, but all the licenses must be adhered to at the same time. See bluecarrot16's explanation here: https://opengameart.org/forumtopic/remixing-by-30-and-by-40#comment-92293

Now for the "probably": Most times, however, there is a single common license that all the source content is compatible with. For instance, if the sources are CC0 and CC-BY, CC0 can be adapted to CC-BY, so the derivative can be under the singular license CC-BY. If the sources are CC-BY and CC-BY-SA, CC-BY can be adapted to CC-BY-SA, so the derivative can be under the singular license CC-BY-SA. Furthermore, many authors authorize the adaption of earlier licenses to later versions, but it depends on the specific assets in question. 

and how is it required that i share them?

CC0, OGA-BY, and CC-BY do not require you to share derivatives, but you are encouraged to do so anyway. CC-BY-SA and GPL require you to have derivatives under the same license. (Different clauses, but the effect is about the same) Provided the assets are accessable via the same avenues as your game, you've met the requirement. For instance, if people can download your game for free, make sure they can access the files in the same download. If you sell your game on itch, provide a link on that game's itch page where people can download the assets for free.

i understand that modifcations of cc-by-sa works have to be licensed under same license, and must be released. but if my modifications are garbage, there is no sense in uploading them to oga, there is no need here for not-useful amateur crappy artwork or trivial derivatives.

"trivial" being the key word there. Trivial changes are not considered derivatives. If your modification is nothing more than a hue shift, or pixel alignment correction, or cropping a sprite, that doesn't need to be "shared-alike". It is sometimes hard to tell if it is truly trivial or if it should be considered a derivative. I can go over some guidelines if you'd like, but when in doubt, share alike.

is having the artwork in the game folder sufficient to be considered "sharing alike?" (with correct identification of the assets as being licensed as cc-by-sa) 

Yes. So long as your players can access the assets your game uses, you've shared them alike. A copy of the assets should suffice, even if it's not the exact specific bits read by the game engine. (it should be an actual copy, though. If you can't swap one file for the other, compile it, and get the exact same results in-game, it isn't a true copy.) Sharing derivatives on OGA is not required to satisfy "share-alike". We certainly like it when you do. However, we ask that you not share derivatives on OGA if those derivatives would never be useful in another game. For instance, deriving a CC-BY-SA asset into your game's custom "Rynosaur Rampage" title screen? You do have to share that title screen as CC-BY-SA, but it doesn't make sense to share it on OGA because no one but you is making a Rynosaur Rampage game.

can someone also try to explain how a mixed asset would be licensed and what i would have to do to satisfy share-alike requirements.

I'd be happy to. Just let me know the cirucmstances if you have any circumstances in addition to your example below:

like, i use a sprite from asset a but i want to take the head of a sprite from asset b and frankenstein it together, then make my own edits to look as good as i am able. but asset a is cc-by 3.0 and asset b is cc-by-sa 4.0. my frankenstein sprites inherits cc-by-sa 4.0 right? 

correct, because CC-BY 3.0 allows for adaptation into CC-BY-SA 3.0 and subsequently CC-BY-SA 4.0

and what if i use a gun from asset c that is cc-by-4.0 and frankenstein it into the drivative from asset a and asset b? now my sprite is derivative from 3 assets with 3 different licenses.

All three are compatible with CC-BY-SA 4.0: 

• Asset A: CC-BY 3.0 can be adapted to CC-BY-SA 3.0
• Asset A.1: CC-BY-SA 3.0 can be adapted to CC-BY-SA 4.0
• Asset B: CC-BY-SA 4.0 doesn't need to be changed, it is the "common license" here.
• Asset C: CC-BY 4.0 can be adapted to CC-BY-SA 4.0
• Derivative: CC-BY-SA 4.0 is common to A, B, C, and your own addtions, assuming you choose to license your own components as CC-BY-SA 4.0

i also have questions about releasing source code to my game. i want to release source code under mit license, with the artwork included in the source and licensed separately as whatever the license requires. can i package them together or do i have to release only code in one zip and only artwork in another zip?

You can release the code under one collection and each set of assets under other collections. Each collection can have it's own license. Projects can contain multiple collections. You do not need to release each part in a separate zip, but there should be some delineation between the collections; some way to tell them apart. I usually release my projects in one zip file, but the zip file contains several folders: 

• Code (MIT)
• Graphics 1 (CC-BY)
• Graphics 2 (GPL)
• Sound (OGA-BY)

i hate to keep asking these questions, i have read the faq multiple times and searched the forums to get answers but i can't make myself confident that how i plan to do things is the correct way of doing it. i'm not worried about getting sued, it's just that ethically i want to do the right thing.

You're doing it right. You've researched it before asking and still have questions. You want to adhere to the law and be ethical. That has earned you unlimited free discussions with the community. (I mean, technically you didn't need to earn them like that; everything here is free, even the answers.) Keep asking questions until your curiosity is satisfied. We will never get annoyed when you do it this way.

also want to thank all who have made awesome artwork and those who have kept this awesome resource going. i've only recently discovered it after taking a 16 year long hiatus from hobbyist gamedev, but i love the spirit of this site. hopefully i can find ways to give back.

Seeing this has made my day. :D