So in the last battle, Venom ends up eating Carnage and killing him (lame and a waste, but Sony gonna Sony) Prior to this there was basically a manhunt for Venom and Carnage. After Carnage is dead Venom and Eddie agree that they will need to hide out. So they go to a remote location in some dingy run down room on a beach. Eddie is watching TV and talking to Venom. The symbiote is basically saying "You wouldn't believe the stuff I've seen travelling across the galaxy all these light years away, but your human mind couldn't handle it."
So Eddie retorts and says he can. Eddie kinda blanks out for a second and the screen is vibrating.
All of the sudden it's interrupted by the room shifting and a whooshing sound. Eddie/Venom are in a similar room but it's much more well lit and you hear JJJ talking about Spider-Man. Eddie is looking around, gets up, looks at the TV.
There's JK Simmons and then Tom Holland Spider-Man unmasked. The symbiote looks at the TV and says I think "That guy..." and then licks the TV. A dude walks out of the bathroom and is like "hey buddy you're in my room"
That's it. Venom is now in the MCU. It makes zero sense other than Sony probably asking Marvel to tie it in so they can make more money.