Venmo has emerged as a popular digital wallet and payment app, providing users with a convenient way to make peer-to-peer money transfers and manage finances. With Venmo's integration of cryptocurrency services in 2021, users can now easily purchase Bitcoin and other cryptocurrencies directly within the app. 

An Overview of Venmo
Launched in 2009 and acquired by PayPal in 2013, Venmo serves as a mobile-based financial service allowing users to transfer funds and make purchases. It operates as a digital wallet where users can store money, make instant transfers, and buy cryptocurrencies.

Venmo introduced the ability to buy, sell, and hold cryptocurrencies in April 2021. It currently supports Bitcoin, Ethereum, Litecoin, and Bitcoin Cash. Users can conveniently access these services right within the Venmo app.

Step-by-Step Guide to Buying Bitcoin on Venmo
Here is a walkthrough of how to buy Bitcoin using Venmo:

1. Open the latest version of the Venmo app and tap the "Crypto" option at the bottom.

2. Choose Bitcoin from the list of supported cryptocurrencies.

3. Select the "Buy" option and enter the amount in dollars you wish to purchase in Bitcoin.

4. Confirm funding source, exchange rate, fees, and other transaction details.

5. Review the order and complete the Bitcoin purchase.

The purchased Bitcoin will appear in your Venmo crypto balance. You can also view historical charts and real-time price changes of Bitcoin within the app.

Transaction Limits and Fees
Venmo imposes weekly and annual purchase limits on crypto transactions. Users cannot buy more than $20,000 worth of crypto per week or exceed $50,000 per year.

Venmo charges fees based on the size of each crypto transaction. This includes a spread on the exchange rate and a transaction fee. Expect fees around 0.5% of the total purchase amount.

Security Risks and Drawbacks
While convenient, transacting in Bitcoin on Venmo has some downsides:

1. Purchases are irreversible and lack fraud protection.

2. Cryptocurrency holdings lack FDIC and SIPC insurance.

3. Bitcoin cannot be transferred out of Venmo to other wallets.

4. Price volatility can lead to sudden losses.

So while Venmo simplifies the process of buying Bitcoin, users should weigh the risks and limitations. Proper research and caution is still advised.