This is Finley from the Social Care team at Shopify.
Welcome to the community! We're glad to have you.
I understand that you'd like your customers to have the ability to select the quantity of the product before they add an item to the cart.
It's also important for your customers to be able to continue shopping on your website after adding the initial product to the cart. Let's go over the next best steps for you here.
Enable a quantity selector
Which theme are you using for your website?
On popular themes like Debut or Brooklyn, you'll see that you can navigate to the Product page of your theme editor and then select the Product pages section on the left-hand side like so:
Use a cart drawer
As for the customers being sent directly to the checkout, what you can do is enable your "cart drawer" if the theme you're using has this. This means that the customer can add a product to the cart and stay on the same page so they can continue shopping.
You can navigate to your Theme Settings in your theme editor and then the Cart drawer like so:
These steps may be the same for your theme, even if it's a third-party theme, so have a look and let me know if you've been able to make that work.
Design your Online Store
While you're working on your theme, you may like to watch our free video course on Shopify Compass, How to Design Your Online Store.
The course goes through Ecommerce design principles, creating your brand identity, and how to apply this to your website.
Let me know how that goes! If you have any issue with the steps above, let me know, and I can take a further look.