Opening a image from Lr into Affinity Photo,

"just export them"- Yes, that is an option I thought of but did not mention earlier.

In an Export dialog you can choose to directly open the image in Affinity if you set Affinity in the 'Post Processing' section of the Export dialog.

Your Export dialog can use all its options for Resize, Rename, File-type, etc, to determine the file characteristics that Affinity will get.

AND you make an Export PRESET that is easily available to Export any image to Affinity. 

With an Export PRESET you can [Right-Click] any image in the library, Choose [Export], select the [Affinity Preset] you created.

"the lastest version of Lr"  And is that "Lightroom" or "Lightroom-Classic"? And which Version Number?

For Lightroom-Classic:  Using Affinity as an Additional External Editor- will [Save] the new image file back to LrC and in the same Folder. IF you use [Save As] in Ps then you can place the new file anywhere but LrC may not know anything about the image unless you Import it to the LrC catalog.

For Lightroom: You would need to Export (Share) the photo to your hard-drive, Open in Affinity, [Save As] it wherever you want. To have the new image back in Lightroom you would need to then Import it to the Lr catalog.