They're special solder sleeves if not clear!
https://www.expresselectrical.co.uk/Catalogue/Terminate-Connect/Heat-Shrink-Solder-Sleeves
I just twist them together and put some electrical tape over - is this significantly better?
Electrical tape turns to a brittle, sticky horrible mess after a few months
Depends on your use case - I connected my rear light with spade connectors because it's on my mudguard and wanted to be able to remove it easily.
If your setup is permanent then and you want it to be waterproof/welded together then the sleeves are a great shout.
@lemonade started
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They're special solder sleeves if not clear!
https://www.expresselectrical.co.uk/Catalogue/Terminate-Connect/Heat-Shrink-Solder-Sleeves