I want to learn how to make up LED strip lighting.

I want to put color changing strips under several shelves, and some other projects. I see kits for sale that can do 2 sections, but where can I get the various cables & connectors to do "several" lengths in parallel, with connecting cables of length I choose? Also would like to attach the connectors to the cables myself, so I can drill a small hole in the back of cabinets & pass the wiring thru.

I have seen strip widths of 8mm and 10mm - how many styles are there? Obviously the connectors are different. Are they all 2 wire?

There are differences in where you can cut a strip - some say every other, some every 9th.

Probably differences in LED density ie LED per inch on the strip.

Also would like to make up "footlights" to illuminate stuff on the mantlepiece. Could use 45 degree wood molding to mount a strip angled upwards.

Most have a 12v brick power supply, I saw one powered off USB (convenient for framing your TV set). Want to make up my own DC power cable length for that if possible to avoid running an AC extension cord.

Is there a good vendor for this stuff that has organized compatible parts for a given type of strip all together?

I see strips powered by 3x AA, probably presumes 1.5v primaries, so maybe would need to use a 4x box for NiMH 1.2V ?