If you only want to validate an idea or want to launch without knowing if it will be profitable, you don't have to pay from your pocket for a majority of cloud products. For the launch of my side project Mindible, I won't be using any paid cloud services. Let me show you how I can do so!
If you have a static frontend, you can use free services like Vercel or Netlify. Their free tiers are really enough for any small launching startup. I would recommend using Vercel since I used it myself for more than a year.
DigitalOcean can give you $100 for free when signing up for the first time to try their products. This way you could use their Droplets or other services for up to 60 days for free. Ideal to host your backend server for free.
If you are using MongoDB, this is for you! I found yesterday that there are tons of credits code (you can found them via Reddit) that you can apply to your account. Using this process, I got more than $600 of credits for cloud-hosted MongoDB: http://prntscr.com/xqvm40
There is also Firebase which is a really powerful cloud database that can help you with real-time features and online/offline synchronization. Their free tier is also pretty big.
Major cloud providers give you free credit and free services when signing up.
Take AWS, Azure or even GCP. Of course, this requires a bit of time to understand how it works, but it's still a good idea for a free Serverless endpoint for instance.