How to define a new theorem in bookdown

Question

I realize that bookdown does not have assumption and hypothesis defined despite being noted here. I wonder how I would define a new theorem environment in bookdown. In particular

1. How do I add the type of cross-referencing for, say, assumption, as mentioned here?

2. How do I define a block for assumption as discussed here? I do not really understand what is written there so I hope to receive some more detailed explanation/solution.

To be more specific, this is my code

---
header-includes:
   - \usepackage{amsmath}
   - \newtheorem{theorem}{Theorem}
   - \newtheorem{lemma}[theorem]{Lemma}
   - \newtheorem{corollary}[theorem]{Corollary}
   - \newtheorem{proposition}[theorem]{Proposition}
   - \newtheorem{definition}{Definition}
   - \newtheorem{example}{Example}
   - \newtheorem{exercise}{Exercise}
   - \newtheorem{assumption}{Assumption}
   - \newtheorem{hypothesis}{Hypothesis}
urlcolor: blue
extension: latex_macros
numbersections: true
output:
   bookdown::pdf_document2:
      toc: false
biblio-style: apalike
---

\newcommand{\lessonNumber}[1]{
   \renewcommand\thetheorem{#1.\arabic{theorem}}
   \renewcommand\thelemma{#1.\arabic{lemma}}
   \renewcommand\thecorollary{#1.\arabic{corollary}}
   \renewcommand\theproposition{#1.\arabic{proposition}}
   \renewcommand\thedefinition{#1.\arabic{definition}}
   \renewcommand\theexample{#1.\arabic{example}}
   \renewcommand\theexercise{#1.\arabic{exercise}}
   \renewcommand\theassumption{#1.\arabic{assumption}}
}

\lessonNumber{1}

\begin{assumption} <!-- I would like to have a label here, for example #trial-->
If A then B
\end{assumption}

Now, let us discuss Assumption \@ref{assump:trial}.