Need help! I'm newbie and can't find leak. Hoses, thermostat changed. Big leak after one of those t connect hose connector was replaced. Any info would be appreciated.

 

Several things can happen, hose not connected, hose clamp in wrong location, rusted or broken fitting.
The metal tubes in the intake can rust and leak. May not leak until you start getting rough with them removing hoses. Sometimes they rust where the hose goes on. Over time, the old hose adapts to the rust. Put a new hose on and it'll leak. Wire brush the tube good before putting the new hose on. If the surface of the tube is badly corroded, you can wipe a thin coating of silicone on the tube before installing the hose. Let it set a few hours before filling up with coolant.
On some cars there is a plastic "T" for the heater core. I had one crack right in front of the flange for the hose. Previous owner had been running cheap "house brand" antifreeze in the car. It had ate the plastic from the inside out. The walls were really thin and brittle. One I touched it, it broke spraying coolant all over the engine. Some parts stores don't sell the "T" fitting. They require you to buy all 3 hoses for $120. Some stores do sell the "T" so check around. Some years used molded plastic clamps on this "T". Others use spring clamps. You can carefully cut the plast molded clamps off but getting the hose off can be a challenge. Its easiest to remove the whole assembly and work on it outside of the car. It may be easier, quicker & cheaper to get a used one from a salvage yard.
Unfortunately they dont make metal versions.