I use 2 from Fdroid, Osmand~ (excellent but probably OTT if you just want altitude) and Organic Maps (a very close second and improving with every update).
Osmand~ can be rigged to show altitude constantly.
In Organic Maps, just press and hold the current location marker and the altitude will be shown at the bottom of the screen.
Thank you once again, Mr. Licaon_Kter, for your reply.
Osmand~ is a huge app. 133.3 MB! Simply huge just for 3 coordinates (lat, long, alt).
Open Street Map Mapviewer however is 13.3 MB, so reasonable.
But in my Linux system I don’t require to install Google maps. I use my web-browser to study the data in the map.
I still am tied more to my laptop. Smartphone I use seldom, that too, only when I require to address certain needs that my laptop can’t fulfill. Like BhimUPI, Rapido, Location, phone calls, clicking rudimentary pics, SMS and other messaging apps.