How to draw a circle with radius from variable?

like this

documentclass[tikz,border=10pt]{standalone}
usepackage{tikz}

begin{document}
defR{3cm}
begin{tikzpicture}
coordinate (A) at (0, 0);
coordinate (B) at ({-R}, 0);
coordinate (C) at (-R, R);
draw (A) circle ({R});
draw (B) -- (C); 
fill[blue] (A) circle (1pt);
fill[blue] (B) circle (1pt);
fill[blue] (C) circle (1pt);
end{tikzpicture}

end{document}

a rough approximation with tkz-euclide

documentclass{article}
usepackage{tkz-euclide}
begin{document}
    
begin{tikzpicture}[]
    %arbitrary points
    tkzDefPoint(0,3){A}
    tkzDefPoint(0,-3){B}
    %tangent to circle
    %mid point of tangent line
    tkzDefMidPoint(B,A)tkzGetPoint{P}
    tkzDrawLine[add=0.8 and 0](A,P)
    tkzDrawPoint(P)
    %perpendicular to tangent
    tkzDefLine[orthogonal =through P](A,B)tkzGetPoint{X}tkzDrawPoint(X)
    %draw circle with radius XP
    tkzDefCircle[through](X,P)
    tkzDrawCircle(X,P)
end{tikzpicture}

end{document}

You can use your syntax almost literally except that you have to add backslashes. I also use circle[radius=R] instead of circle (R), but the latter would work, too, and use a loop for the blue circles.

documentclass[tikz,border=3mm]{standalone}
begin{document}
begin{tikzpicture}[declare function={R = 4;}]
coordinate (A) at (0, 0);
coordinate (B) at (-R, 0);
coordinate (C) at (-R, R);
draw (A) circle[radius=R];
draw (B) -- (C); 
foreach X in {A,B,C}
{fill[blue] (X) circle[radius=1pt];}
end{tikzpicture}
end{document}