Like lots of previous shooters from the past 5 or so years, this one also was radicalized in far right extremist online spaces, mostly the Terrorgram channels in Telegram and their offshoots. The symbols on the weapons call out previous member shooters and use catchphrases from Terrorgram like "bowl patrol" (meaning Dylan Roof fans/emulators). This shooter was highly likely to also be a member of a 764 network group, probably the nihilistic "True Crime Community" or TCC fans based on what I've seen so far. This group's focus is on lauding past mass murderers and cultivating their qualities in their members.
If anyone wants to know what the internet is actually like now underneath the surface, this is just one of many samples of what's already here and what the AI future is going to supercharge.
Edit:
A somewhat OK overview of the groups it's likely the shooter was deeply enmeshed with