How to create a frame for tikzpicture?

Is this what you want to achieve?

The matrix is a node, you could draw its border, without adding external frames.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{fit, backgrounds, matrix, arrows.meta}
\tikzset{
    startstop/.style={
        rectangle, rounded corners, minimum width=2cm,  
        minimum height=1.5cm,text centered, draw=black, fill=RYB2
    },
    process/.style={
        rectangle, minimum width=2.5cm, minimum height=1cm, text centered, draw=black, fill=RYB1
    },
    arrow/.style={
        blue,-{Stealth[length=6pt]}
    },  
    dasharrow/.style={
        blue, dashed,-{Stealth[length=6pt]}
    }
}

\definecolor{RYB1}{RGB}{218,232,252}
\definecolor{RYB2}{RGB}{245,245,245}

\begin{document}
    \begin{figure}\centering
        \begin{tikzpicture}[font=\tiny]
        % blocks
        \matrix[matrix of nodes,
        draw, inner xsep=6pt,inner ysep=20pt,%frame 
        row sep=4ex,
        column sep=5.4em,
        nodes={anchor=center},
        column 2/.style={nodes={process}},
        ] (m) {
            & Text &&[-3.5em]\\
            |[inner sep=0pt]|\includegraphics[width=2cm]{example-image-a} & Text &&\\
            & Text & |[startstop]| Text & |[inner sep=0pt]|\includegraphics[height=.5cm]{example-image-b} \\[7ex]
            & Text &&\\
            & Text &&\\
        };
        % block background
        \begin{scope}[on background layer]
        \node [draw, fit=(m-1-2)(m-3-2), fill=cyan!5, inner sep=10pt] {};
        \node [draw, fit=(m-4-2)(m-5-2), fill=cyan!5, inner sep=10pt] {};
        \end{scope}    
        % vertical arrows
        \foreach[evaluate=\myblock as \mysucc using int(\myblock+1)] 
        \myblock in {1,2,...,4}
        {\draw[arrow] (m-\myblock-2) -- (m-\mysucc-2);}
        % horizontal arrows
        \coordinate (a) at ([yshift=2ex]m-2-2.west);
        \draw[dasharrow] (a) -- (m-2-1.east|-a) node[midway, above]{Text};
        \coordinate (b) at ([yshift=-2ex]m-2-2.west);
        \draw[dasharrow] (m-2-1.east|-b) -- (b) node[midway, above]{Text};
        \coordinate (c) at ([yshift=2ex]m-3-2.east);
        \draw[dasharrow] (c) -- (m-3-3.west|-c) node[midway, above]{Text};
        \coordinate (d) at ([yshift=-2ex]m-3-2.east);
        \draw[dasharrow] (m-3-3.west|-d) -- (d) node[midway, above, align=center]{Texttext\\texttext};
        \draw[dasharrow] ([yshift=4ex]m-3-3.east) -| (m-3-4);   
        \draw[dasharrow] (m-3-4) |- ([yshift=-4ex]m-3-3.east); 
        \end{tikzpicture}
    \end{figure}    
\end{document}

A frame can also be set inside tikzpicture at the end, when the bounding box is known:

\documentclass[tikz]{standalone}
\usetikzlibrary{ducks}
\begin{document}
  \begin{tikzpicture}
    \draw[thick, mark=*]
      (0, 0) -- (1, 2) -- (2, 0)
      (.4, .8) -- (1.6, .8)
    ;
    \fill[radius=2pt, red]
      (0, 0) circle[]
      (1, 2) circle[]
      (2, 0) circle[]
      (.4, .8) circle[]
      (1.6, .8) circle[]
      (.9, .8) pic[scale=.1]{duck}
      (.9, 0) pic[xscale=-.1, yscale=.1, /duck/water]{duck}
    ;

    % Frame
    \def\sep{\fboxsep}
    \draw[thick, blue!75!black, double=yellow]
      (current bounding box.south west) ++(-\sep, -\sep) coordinate (ll)
      (current bounding box.north east) ++(\sep, \sep) coordinate (ur)
      (ll) rectangle (ur)
    ;
  \end{tikzpicture}
\end{document}

It seems that nobody mentioned show background rectangle option which I think it's the easier solution.

Background library is usually mentioned to draw on background layer, but it also provides show background rectangle which can be used to draw a frame around the tikzpicture. You can define a separation between borders and frame, and also the used style.

Following code shows the default solution:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{fit, backgrounds, matrix, arrows.meta}
\tikzset{
    startstop/.style={
        rectangle, rounded corners, minimum width=2cm,  
        minimum height=1.5cm,text centered, draw=black, fill=RYB2
    },
    process/.style={
        rectangle, minimum width=2.5cm, minimum height=1cm, text centered, draw=black, fill=RYB1
    },
    arrow/.style={
        blue,-{Stealth[length=6pt]}
    },  
    dasharrow/.style={
        blue, dashed,-{Stealth[length=6pt]}
    }
}

\definecolor{RYB1}{RGB}{218,232,252}
\definecolor{RYB2}{RGB}{245,245,245}

\begin{document}
\begin{figure}\centering
\begin{tikzpicture}[font=\tiny,show background rectangle]
    % blocks
    \matrix[matrix of nodes,
        row sep=4ex,
        column sep=5.4em,
        nodes={anchor=center},
        column 2/.style={nodes={process}},
        ] (m) {
        & Text &&[-3em]\\
        |[inner sep=0pt]|\includegraphics[width=2cm]{example-image-a} & Text &&\\[-2ex]
        & Text & |[startstop]| Text & |[inner sep=0pt]|\includegraphics[height=.5cm]{example-image-b} \\[7ex]
        & Text &&\\
        & Text &&\\
    };
    % block background
    \begin{scope}[on background layer]
        \node [draw, fit=(m-1-2)(m-3-2), fill=cyan!5, inner sep=10pt] {};
        \node [draw, fit=(m-4-2)(m-5-2), fill=cyan!5, inner sep=10pt] {};
    \end{scope}    
    % vertical arrows
    \foreach[evaluate=\myblock as \mysucc using int(\myblock+1)] 
        \myblock in {1,2,...,4}
        {\draw[arrow] (m-\myblock-2) -- (m-\mysucc-2);}
    % horizontal arrows
    \coordinate (a) at ([yshift=2ex]m-2-2.west);
    \draw[dasharrow] (a) -- (m-2-1.east|-a) node[midway, above]{Text};
    \coordinate (b) at ([yshift=-2ex]m-2-2.west);
    \draw[dasharrow] (m-2-1.east|-b) -- (b) node[midway, above]{Text};
    \coordinate (c) at ([yshift=2ex]m-3-2.east);
    \draw[dasharrow] (c) -- (m-3-3.west|-c) node[midway, above]{Text};
    \coordinate (d) at ([yshift=-2ex]m-3-2.east);
    \draw[dasharrow] (m-3-3.west|-d) -- (d) node[midway, above]{Text};
    \draw[dasharrow] ([yshift=4ex]m-3-3.east) -| (m-3-4);   
    \draw[dasharrow] (m-3-4) |- ([yshift=-4ex]m-3-3.east);
\end{tikzpicture}
\end{figure}    
\end{document}