I need an ELI5 for LCSs

[GCAT01]

I am trying to understand why you would choose particular locations for an LCS on an object when assembling. Most tutorials just say "put the LCS here" but never explain why.
I understand this is where the attachment between two pieces is centered but I have some questions:

• Are two pieces "joined" by having the origin of their respective LCS overlap (coincident)? Is this how this works? I'm guessing from videos and tutorials but I haven't found anywhere that explicitly states this.
• I assume parts that rotate around something will need the LCS centered around the point of rotation in both parts?
• Is it better to place the LCS on an edge? Midway through the joining bodies? Does it matter?
• Does LCS orientation ultimately matter? In your Tutorial 1, one of the bearings came in sideways and you have to rotate it around an axis to get it to line up. Will this ultimately affect anything?
• If you have multiple secondary parts all attached to a "main body" or "base" does it matter which parts have the LCS. For example, if I had a box, do I need an LCS for every single side with adjacent sides AND each edge of the top/bottom?
• To be completely clear, you need a pair of LCS for each "connection" in the assembly. Like a dot of glue that says "this piece and that piece connect here".

Sorry if some of these are general CAD questions. This is my first time using any CAD software and I would like to learn to do things the "right" way the first time. Your tutorials have been really nice and helpful so far! Thank you for making them.

[Zolko-123]

Are two pieces "joined" by having the origin of their respective LCS overlap (coincident)? Is this how this works?

yes (actually, not only the origins but also the axes).

Does LCS orientation ultimately matter?

No, you can re-orient the parts during placement

I assume parts that rotate around something will need the LCS centered around the point of rotation in both parts?

yes. If you have an axis and a hole, place the 2 LCSs in the 2 parts centered on the axis and the hole

To be completely clear, you need a pair of LCS for each "connection" in the assembly

exactly. The particularity of Assembly4 is that you define these "connections "in the part itself, and in the assembly you only make the joining – pairing – of those connections. The idea is that a part "knows" best where it can be connected, since in the real-world you never glue 2 objects at random places. Whether they are edges or holes or faces or midpoints depends on the part.