What happens to (good) posts when a user is deleted, and can I find them by
userid, either on site or via SEDE?
When a user is deleted, all their negatively scored posts are deleted along with them. You won't be able to find these in SEDE, nor on the site if you don't have 10k rep.
Their positively scored posts are left intact. These you will be able to find in SEDE, using a query similar to the following:
SELECT Id as [Post Link] FROM Posts WHERE OwnerDisplayName = 'user5098';
Substitute 5098 for the target user ID.
When a user is destroyed, all their posts are deleted regardless of their score, so those won't show up in SEDE any longer.
When you don't know the userid you'll need to do a bit of forensic work to know to which ownerdisplayname the posts will now belong. You do need to know the/any username the user used on the site and they need to have been a bit active in comments.
Start with a search in Comments to find posts where the user was @-mentioned, for example:
select id as [Comment Link] from comments where text like '@H2CO3%'
When you visit some of the comments, you'll find earlier comments that were made by the user you're looking for. Note their displayname.
You can now run:
select id as [Post Link] from posts where ownerdisplayname = 'user529758'
to find the posts that were positively scored when the user was deleted and still visible today.