Artisto
On your phone, click on the .apk file under Assets to download it, then open the apk file on your phone (you can either click open on the notification that pops up, or you should find it in your downloads) and it will prompt you to install the app. A screen will show that asks for permission to install unknown apps, you can allow this and then disable the permission again after the app is installed for security purposes.
Also to note, if you download apps this way, you won't get auto updates or anything like you would with Fdroid. Instead, you can use something like Obtainium which you install the same way as above. To use Obtainium, you'll need to copy/paste the URL for the Releases page of the app (e.g. the link I provided in my previous comment) on the Add App tab at the bottom of Obtainium.
Obtainium is easier to use than it might initially seem, but if its not convenient enough then you can go ahead with Fdroid which can just be installed by clicking the Download button on Fdroid's website.