I have a AGFA Billy Record 4.5 focusing rin...

They were made in Germany from around 1935 to around the mid 1950's and were never supplied with a user manual as such. Just a flyer with a brief description of the camera and controls and it was all in German. Often there was no flyer supplied at all.

There's not really much need: opening and closing the camera is easy enough and you've already clearly done that, loading and winding the 120 roll film is straightforward. Other than that you have a shutter c0cking lever, four shutter speeds (B, 1/25th, 1/50th and 1/100th), three or four apertures depending on the exact lens fitted and the focus ring with distance scale. Exposure is determined either by using an external exposure meter or by using experience and the Sunny Sixteen Rule. There is no in built light meter so there is no need for any film speed control. Although it's a 120 roll film camera, it takes 6 x 9cm exposures so you only get eight shots per roll and the lenses were never really designed for colour film and give best results if you stick to black and white. Early (pre-1940) versions lacked double exposure protection so this is either a creative bonus or an annoyance depending on your point of view. As you'll have already discovered, the camera has what's called a "waist finder". This was a popular type of viewfinder where you hold the camera at waist level and look down into the viewfinder.

The stuck focus ring is almost definitely due to dried out, gummed up, lubricant. If you leave the camera in a warm car for a day (not direct sunlight) it might soften the gunge enough to allow you to gently but firmly try to turn the ring, but you do risk damaging it irreparably. It would be far more sensible to dismantle the lens and overhaul it. There's not a lot of anything much in there and the camera may be old but is not valuable so you have little to lose by trying this for yourself. Even if it goes wrong you'll have gained experience for future use.

Hope you have fun with the camera, they're always best when being used rather than stuck inside a collector's cabinet. If you've found my posting to be of help then please return the favour by rating my answer.