Checked with my Firefox - when I go to Telegram FOSS | F-Droid - Free and Open Source Android App Repository
scroll down to Version 7.3.1 (22065) and then use the “Download APK” link
Download APK 30 MiB PGP Signature | Build Log
I get the org.telegram.messenger_22065.apk file:
SHA256 (org.telegram.messenger_22065.apk) = b0fd1d299f580d2319bd181048c3c2f3f6ef6135feadd582252828c68de654e1
The URL to the APK is https://f-droid.org/repo/org.telegram.messenger_22065.apk
What you can do is to go to the Download Manager of Firefox and find your repo.apk on the list of the recent downloads, right click and select “Copy Download Link” and then we can identify the source of that.