>>1872030
Basically, you farm various game modes that give you "secret character letters", then you open said letters by doing a special mission. This gives you character shards, which can be traded for a copy of a character once you have 30 shards. It seems like an hour or so of farming can give you enough shards for 1 character copy.
Getting weapons seem to roughly work the same way. So yeah you could max out every character and weapon by just grinding. Also newly released characters and weapons will be made easier to get with a special gamemode for them.