I am trying to connect two android devices, one with version 12 and the other with a lower version.
I have two doubts.
firs of all, where can I get the APILEVEL1, to know the andorid version?. And another question is, in the bluetooth server, do I have to do the same?
Hello Jose, welcome to the Forum 
You should open your own Topic as your question is not related to this Topic, so I have created one for you.
Usually, people know the Android version but not the API Level 
On my website:
ProfessorCad: Tips & Tricks Android Version and Related API Level
To give you good advice, we need to know the Bluetooth version of the devices - is one of them a Smartphone? Tell us what the devices are.
Thank you very much for you answer, I inform you that our smartphones are two Xiaomi Redmi 9, one of them running MIUI 12.5.8 (android11). Another with MIUI 13.0.1 (Android12). They have to act indistinctly as a server or as a client.
Hi again
OK, and is it currently possible to use Bluetooth to 'pair' the two phones to exchange data or is it the case that Xiaomi simply don't provide that facility?
Can you tell us the Bluetooth version of each phone?
Frisrt of all, thanks you for your help. I am asking you about the API or extension for app inventor for to know the android versión, as explained in this entry "Bluetooth no se conecta con versiones 12 de Android - #3 by ChrisWard"
There are a number of videos online that will help you get familiar with Bluetooth on the Xiaomi Redmi 9:
..and how to transfer data from one Xiaomi to another Xiaomi:
Transfer Data from Xiaomi to Xiaomi: A Comprehensive Guide.
The Android version is not so much a concern, what we need to know is the Bluetooth version as there are technically two 'types' of Bluetooth, 'Classic' and 'BLE' (Bluetooth Low Energy). Which version of Bluetooth used is entirely down to the Smartphone manufacturer, regardless of the Android Version.
There are a number of versions of the Xiaomi Redmi 9:
Xiaomi Redmi 9
Xiaomi Redmi 9A
Xiaomi Redmi 9T
Xiaomi Redmi 9C
.... and each of those has further differentiation, so you must determine, from your exact Redmi models, what the Bluetooth version is. Some phones actually state this in their settings, but my Huawei P30 Pro for example, does not!
However, I was able to find out here:
Thank you very much for the information, I am going to try solve it