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 ## 🩸 Bitcoin Bridge
 
 * [What is the Bitcoin Bridge](bitcoin-bridge/what-is-the-bitcoin-bridge.md)
+* [Understanding the Bitcoin Bridge](bitcoin-bridge/understanding-the-bitcoin-bridge.md)
 
 ## 🌉 Cross-chain Bridge
 
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+# Understanding the Bitcoin Bridge
+
+## Custody of assets and monitoring of reserves
+
+Bitcoin Bridge is custodial in that BTC and BRC20 tokens being bridged are held in a Multisig wallet on Bitcoin. Users can monitor the reserves real-time at [https://app.alexlab.co/bridge-reserve](https://app.alexlab.co/bridge-reserve).
+
+### BTC Multisig
+
+{% embed url="https://mempool.space/address/bc1pylrcm2ym9spaszyrwzhhzc2qf8c3xq65jgmd8udqtd5q73a2fulsztxqyy" %}
+
+### BRC20 Multisig
+
+{% embed url="https://mempool.space/address/bc1p06r44ervnukj3kxnqt863sz9hly5m7f80k7l94aplnd6z2tnrzvstdkzsq" %}
+
+### Latest circulating supply of aBTC
+
+You can check the latest circulating supply of aBTC or other L2 BRC20 tokens by calling `get-total-supply` function of the relevant contract (please note the number is in 8-digit fixed notation, i.e. the last 8 digits represent the decimals). 
+
+Alternatively the same is also available at, for example, for aBTC
+
+{% embed url="https://api.alexlab.co/v1/stats/total_supply/token-abtc" %}
+
+## Design overview
+
+<figure><img src="../.gitbook/assets/image (6).png" alt=""><figcaption></figcaption></figure>
+
+Bitcoin Bridge also is bi-directional or “two-way” bridge, meaning you can freely transfer assets between Bitcoin and Stacks and vice versa.
+
+On Bitcoin, users interact with Multisigs (operated by ALEX LAB Foundation) to lock assets to be bridged ("source asset"), and on Stacks to receive the L2 asset ("destination asset").
+
+On Stacks, users interact with Endpoints to burn the L2 asset ("source asset") and on Bitcoin to receive the L1 assets ("destination asset").
+
+Asset transfers from users to [Multisigs](understanding-the-bitcoin-bridge.md#custody-of-assets-and-monitoring-of-reserves) and [Endpoints](understanding-the-bitcoin-bridge.md#endpoints) are monitored by a group of "[relayers](understanding-the-bitcoin-bridge.md#relayers)", who calls into Multisigs and Endpoints to trigger the transfer of the received destination assets to the relevant address.
+
+Additionally, users on Bitcoin may provide additional data (`OP_RETURN`) to trigger certain smart contract interaction on their behalf automatically by Bitcoin Bridge.
+
+### Endpoints
+
+Endpoints are the smart contracts that handle the asset transfers. They are owned by multisig contracts ([Executor DAO](https://explorer.stacks.co/txid/0xf4bd95ea0486e6a50ae632c613f1d72b2a5bbbc4211b494cd0f1d3443658544d?chain=mainnet) on Stacks) operated by ALEX LAB Foundation.
+
+That the assets are held by contracts owned by multisig contracts is important because this minimises the risk of a malicious actor stealing the private key and assets sent by users.
+
+CoinFabrik audited the Endpoints.
+
+* Bridge Endpoints: [https://cdn.alexlab.co/pdf/ALEX\_Audit\_202310\_Bitcoin\_Oracle\_and\_Bridge.pdf](https://cdn.alexlab.co/pdf/ALEX\_Audit\_202310\_Bitcoin\_Oracle\_and\_Bridge.pdf)
+
+### Relayers
+
+Relayers are responsible for triggering the destination asset transfer process.
+