Hi WAM,
Thanks for choosing Microsoft Product. There was tech support spam posted to this thread. The post at Microsoft Community was deleted, but you may have received a copy of the spam in a Notification Email. Do not call whatever phone number was posted which does not belong to Microsoft.
Per your description, you would like to search the lookup field to get the from another List A, but alternatively have to scroll down the whole dropdown list until getting the required record.
If so, I’ve test it on my side in SharePoint online, while I get the same situation as yours. There seems no option for searching the specific column only have to scroll down.
Therefore, to further help you, we highly recommend you go to the related SharePoint UserVoice: Lookup on lists over 5,000 that multiple users have share their similar concern in that platform, actually UserVoice is the Official channel for the SharePoint team to collect more good ideas from users which help improve our product, that’s why we sincerely suggest you go there to let the team aware such feature is really needed and beneficial.
(If I misunderstand your scenario, please point out, we are willing to further investigate your issue.)
Besides, any member who has some new ideas are welcomed to post here.
And your understanding is highly appreciated!
Wish you a nice day!😊
Best regards,
Gloria