Reddit reviews Kuject 120PCS Solder Seal Wire Connectors, Self-Solder Heat Shrink Butt Connector Solder Sleeve Waterproof Insulated Electrical Butt Splice Wire Terminals for Marine Automotive Boat Truck Wire Joint

They are all over Amazon and work surprisingly well. I was skeptical at first, but as long as you make a decent mechanical connection with the wires first they work great.

To add to what paxtana said, if you plan on riding in wet conditions I'd recommend using some waterproof heat shrink connectors like these. In my experience, the silicone covers that come preinstalled on those connectors don't really hold up long term in wet conditions and can sometimes vibrate loose to expose the metal they're supposed to protect. I've personally used these on all of my builds and have never had them fail.

I would actually get these: (Real 18AWG 43x2pcs Strands) 10 Pairs DC Power Pigtail Cable 12V 5A Male & Female + 10 Pairs DC Power Jack Plug Adapter Connector for CCTV Home Security Surveillance by MILAPEAK (2.1mm x 5.5mm) https://www.amazon.com/dp/B07CWQPPTW/ref=cm_sw_r_cp_api_i_LmT0Db7G21FSE

I like them better because there are less exposed wires and they are higher quality for pretty much the same price.

To connect a single wire to two things, you split the wire. Take a higher gauge stranded wire, strip it a fair amount (a couple inches) then take half the conductors and twist them together, same with the other half. Use heat shrink so now you’ve got one wire that splits into two wires. Solder additional lengths of wire to your split ends if you need the individual split sections to be longer.

Protip: use these to make soldering wires together dead simple. Kuject 120PCS Solder Seal Wire... https://www.amazon.com/dp/B073RMRCC3?ref=ppx_pop_mob_ap_share

Buy yourself a heat gun if you think you’ll do this more than ~5 times, it makes soldering wires together a 30 second task, very easy.

These are what I used