The only way to acquire apex is through beating the Apex Raid, once you beat the raid with your team you get an incubator that gives you a portion of its dna(depends on level, but its takes 300). To beat the apex raids, you will need to have assistance from 3 of your friends or alliance mates using a total of 4 creatures(1 creature chosen by each player), and you preferably want to use a good strategy that will make it go quicker and give you a decent chance at winning as long as you have creatures with proper requirements. Each raid is different btw, mortem rex is 2 rounds, lux is 3, gorgotrebax is 4, cera 3, and I think the dilo is 3. There are good alliances with good strats and raid-mates, though I’m not sure which you can get into as each appears to have different requirements to join.
That’s my best explanation, others can probaby do better, or you can read the release notes for when raids were introduced.