\documentclass[tikz]{standalone}
\usetikzlibrary{arrows.meta, bending, angles, decorations.pathreplacing}

\begin{document}

\begin{tikzpicture}[
x=4cm, y=4cm, >=Triangle,
bracy/.style={decoration={brace, amplitude=2mm}},
dec/.style={decorate, draw, solid}
]

% Points P, Q, R, and the origin O.
\coordinate[label=below left:$O$]          (O) at (0,0)
coordinate[label=above right:{$P(x,y)$}]  (P) at (60:1)
coordinate[label=below right:$Q$]         (Q) at (P|-O)
coordinate[label=below right:{$R(1, 0)$}] (R) at (1,0);

% Points for X and Y axis.
\coordinate (x-) at (xyz cs: x=-1.1)
coordinate (x+) at (xyz cs: x=+1.1)
coordinate (y-) at (xyz cs: y=-0.1)
coordinate (y+) at (xyz cs: y=+1.1);

% Section of unit circle corresponding to angle theta.
\filldraw (R) [fill=blue!50!white, draw=white] arc [start angle=0,end angle=60, radius=1] -- (O) -- (R);

% Rectangle triangle PQO.
% \filldraw (O) [fill=blue!20!white] (P) -- (Q) -- (O);

% Section of (the upper half of) the unit circumference that does NOT
% correspond to angle theta.
\draw (-1, 0) arc [radius=1, start angle=180, end angle = 60];

% Section of unit circumference that corresponds to angle theta.
\draw (P) [blue, very thick] arc [radius=1, start angle=60, end angle = 0];
% \draw (R) [red, very thick, domain=0:120] plot ({cos(\x)}, {sin(\x)});

% Dotted label to indicate the length of the arc corresponding to angle theta,
% is also theta.
\draw (1.03, 0) [dotted, to-to] arc[start angle=0, end angle=60, radius=1.03]
node[midway, fill=white, above=1mm, right=0.5mm] {$\theta$};

% Mark points P and R.

% Line from Q to P, with decorations.
\draw (P) -- (Q) [postaction={dec, bracy}]
node[midway, right=2mm] {$y =$}
node[near end, above = 5mm, right=2mm] {$\sin \theta$};

% Line from O to Q, with decorations.
\draw[densely dotted] (Q) -- (O) [postaction={dec, bracy}]
node[midway, below=2mm] {$x=\cos \theta$};

% Line from O to P.
\draw (O) -- (P) ;

% Actually draw X and Y axis.
\draw[->] (x-) -- (x+) ;
\draw[->] (y-) -- (y+) ;

\pic[draw, black, thick, ->, pic text=$\theta$, angle radius=7mm] {angle=R--O--P};
\end{tikzpicture}

\end{document}