There is no way to set the default Line Cap/Join attributes of a Shape Layer.
You can however copy attributes from one Shape onto other shapes. You can do this productively by –
(1) selecting the source attribute(s) and copying
(2) select target attributes and then paste
To speed up (2), use the Timeline’s Search box to type in your keyword – Line for your specific case. Select the layers you want to be part of the search (not necessary if you want to search across all layers in the Timeline) and then type your keyword. Then manually select the attributes and paste.
All of AE’s Brush Tools are applied in the Layer Panel; not the Composition Panel. You can read the documentation for all the effects under the Help Menu in AE – select Effect Reference.
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