Disclaimer: this is not technical advise nor financial advise, messing with bootloader, custom recovery and OS roms can pose some risk.
What is a Custom Rom?
When you buy an Android phone you don't get the pure android OS but a customized version of the Android Operating Software by the company that made your phone. Unless, you buy a Google Pixel or Nokia or Motorola phone.
If you buy say a OnePlus phone then you would most likely be getting OxygenOS which is a customized version of Android made the OnePlus for their phones.
Sometimes it will come with a lot of bloatware, i.e. apps you can't uninstall and getting keeping you ads notifications. However you have the option to change your operating system from what the company installed to something else.
Difference between custom rom and rooting
Not many people know this but you don't need to root your phone to install a new OS through custom rom on your android phone.
Rooting gives you the superuser access and can access root folder of your phone, this gives you more control over your device but will also void your warranty.
However, to install a different OS on your root you can do it without rooting.
For this you need to first unlock your bootloader
Then install a custom recovery
And then flash the OS rom on your phone
Can you install any OS on any Phone?
No, firstly some maker do not want you to install different OS on your phone so they will not let you unlock the bootloader of your phone, like the IQOO or Samsung.
Even if people find a way to unlock it then company may come up with a update to remove that method.
Secondly, even if you unlock your phone the custom rom can be specific to the device. So the person developing that custom rom may need to release a version compatible to your device.
And not all phone are made equal so some phone may have a large community that is constantly updating and developing on the device, while some some that the manufactures themselves discourage may not have such community.
Best place to check the community support or to find any solution for your phone is to check on XDA forum.
Customized Android phones as Crypto wallets!
Some of the roms provide better privacy and security then your normal out of the box OS as they useful transfer a lot of user data to the manufacture and Google.
Therefore, privacy advocate do not like Google services which run on every Android device out of the box.
Many of the custom roms have Google services and the Google account but there are some de-Googled roms out there as well like LineageOS and GrapheneOS.
Note: LineageOS may run on many phone models but they still run the original manufacture's firmware underneath, whereas GrapheneOS is said to be most secure but can only be installed on Google Pixel phones.
De-Googled phone
The De-Googled phone will not have the google services therefore you can run apps that depend on Google services, i.e. Google Play Store.
So where will you get your apps from?
F-Droid is one option: you can download the apk from their website and then install it on your phone. F-Droid supports a lot of open source apps and you can easily find an app for your needs.
Crypto wallet apps?
Many of the wallet apps you use may not be available on F-Droid, so what can you do?
One option is to go directly to the official website and find their GitHub page from where you can find the apk for you to install on your phone. Many of the wallet developers also give the option to download the apk directly from their website.
Is installing apps from apk outside the app store risky?
Yes, it is and you should be careful while installing any app. However that is also true when you are installing an app from the Play store as well. Just search for apps banned from Play store and you will find many harmful apps that made their way into the Play store.
However, if you know the source of the apk to be legitimate and manage the permission you give to a particular app then you can be relatively safe.
If you want to install say the Bitcoin.com wallet then you can head over to their website, go to the wallet section, right below the option for App Store & Play Store there would be an option called Other versions, click on that and it will take you to their GitHub page their you can click on the latest version and from their under the assets option you will find the apk to download.
Final Thought
Only mess with BootLoader, Custom Roms and Rooting if you understand what you are doing and how you should do it.
That being said their is a large number of sources on the internet to learn about all this, search on YouTube and their will a some many videos explaining everything in detail. xda forums are also a great place for finding answers to your question.
Learn about Customizing and Rooting an Android phone and when you are confident enough then first start with a spare phone and start experimenting.
It is a great community out there that gives you more control over your device.
Happy Flashing
