Easy:
In the PureOS store, using the magnifying glass icon, search for “screen shot” (two words) to find and install Purism’s own L5-adapted application: List of Apps that fit and function well [Post them here.] - #258 by amarok
Then search “screen recorder” to find and install Screen Recorder: List of Apps that fit and function well [Post them here.] - #260 by amarok
(Also choose a video player to work in conjunction with that, as described in the link.)
By the way, in the software store, in the Preferences, you’ll have to enable “Show Incompatible Applications” - a misnomer, actually, as many (but not all!) PureOS apps outside Purism’s curated selections for the L5 will work fine, or well enough, on the L5.
Sometimes it takes a while for these to populate, as it has to load the complete catalog over the network from Purism’s repository each and every time. (My Linux Mint computers also have such a delay loading from the Mint repo.)