As indicated on the Oneplus 5T-installation page, you are required to install TWRP. As stated in “Installing /e/ from custom recovery Step 3.” you can create a backup with TWRP.
Recently I created a backup of my /e/-Nougat on LeEco Le2 and restored it afterwards and ran into a few problems, so I recommend to consult the documentation and tutorials about TWRP.
There are apps on Google-Play that can only be accessed after you purchase them. Apps like that require an google account, so you would need to submit your Google-account details via Yalp.
Google provides some services and infrastructure for app developers to utilize.
Since /e/ tries to reduce the amount of data, that is leaked to Google, they replaced some of these services and infrastructure by the open source project MicroG. MicroG is however not in all aspects compatible with the Google services, so there are apps that be installed, but not used on /e/. If you happen have problems with such an app, you can indicate such apps here on the forum.
As far as I know, otherwise all apps are supposed to be running on /e/.
/e/ is still a very young and evolving OS and still in beta phase, so it can be expected that there are several issues and inconveniences present. You can have a look at the known issues at https://gitlab.e.foundation/groups/e/-/issues. But all basic functionalities are supposed to be working.
I believe these were not meant to be there. 