Thanks for everyone who posted an answer here. By reading all answers, I think the solution depends on your application

1. You can manipulate the text directly and just want to make part of the text bold/italic etc.

Than the @mahan's solution would be the best

Text("**Is this part of the text bold?** Yes, it is.")

2. You can NOT manipulate the text directly and you want to make change to the color, font other than boldness

E.g. All the text is retrieved from backend or there are some other middle layers between the source: text and destination: view Inspired by the solution by @dip, I have extension to View

extension View {
    func hightlight(
        text: String,
        textFont: Font,
        textColor: Color,
        highlight: String,
        highlightFont: Font,
        highlightColor: Color
    ) -> AttributedString {
        var attrText = AttributedString(text)
        attrText.font = textFont
        attrText.foregroundColor = textColor

        if let range = attrText.range(of: highlight) {
            attrText[range].font = highlightFont
            attrText[range].foregroundColor = highlightColor
        }

        return attrText
    }
}

Then to use it in this way

            Text(hightlight(
                text: text,
                textFont: .system(size: 18, weight: .regular),
                textColor: .green,
                highlight: highlightedText,
                highlightFont: .system(size: 18, weight: .bold),
                highlightColor: .red)
            )

Here is the full example and final effect

//
//  ContentView.swift
//  LearnSwiftUI
//
//

import SwiftUI

struct ContentView: View {
    let text = "A very long text and we need to the following text to bold. Is this part in bold? Yes, it is."
    let highlightedText = "Is this part in bold?"
    
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundStyle(.tint)
            
            Text("A very long text and we need to the following text to bold. **Is this part in bold?** Yes, it is.")
            
            Text(hightlight(
                text: text,
                textFont: .system(size: 18, weight: .regular),
                textColor: .green,
                highlight: highlightedText,
                highlightFont: .system(size: 18, weight: .bold),
                highlightColor: .red)
            )
        }
        .padding()
    }
}

#Preview {
    ContentView()
}


extension View {
    func hightlight(
        text: String,
        textFont: Font,
        textColor: Color,
        highlight: String,
        highlightFont: Font,
        highlightColor: Color
    ) -> AttributedString {
        var attrText = AttributedString(text)
        attrText.font = textFont
        attrText.foregroundColor = textColor

        if let range = attrText.range(of: highlight) {
            attrText[range].font = highlightFont
            attrText[range].foregroundColor = highlightColor
        }

        return attrText
    }
}