I've mainly done this with hashtags, so I may be off if it differs somewhat for keywords, but I'm pretty sure it works as a logical OR.
So if I put a bunch of hashtags in a filter for sports like #sports, #football, #hockey, etc. if a post contains any one of those tags, it should filter it for me. The nice part is you can adjust it so that it either completely hides the filtered results so they don't clutter up your feeds, or hide with warning and the name of the filter on the warning notice, which allows you to click & review the post to see whether you really wanted it caught by your filter or not.
If you're just starting out you'd probably want to run it with warnings first to get a sense of whether it's working as desired, and if not adjust as needed.
Thankfully folks tend to be pretty respectful of spoiler concerns and will specifically say spoilers below when writing about them, use content warnings to hide them, and use hashtags like #[GameName]spoilers, which can help when making filters. It's not ideal, but if I really don't want to see spoilers for something, I'll filter by its name and hashtag and set it not to expire (you can set filters to last only a week if you like, so you don't forget to disable them).