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Roger,
Good questions. The help you are looking for is here on the site.
First, I try to spend time everyday playing standards in various keys.
For example, Rolling In My Sweet Baby’s Arms, I work on playing it in popular keys A, D, G, C but also add B, Bflat and E.
After you play tunes in the “common keys” a few times you’ll realize you can transpose to any key.

The video lesson you are after is “Nashville Numbering System”, by Megan Lynch Chowning. The whole series is great with simple theory. In particular in the first Nashville Numbering lesson video she describes the “Fiddle Capo”.
That lesson will get you playing in all the keys with chord accompaniment.

It is impressively simple but not really easy.

Best luck,
George