The "Merge By ID" option in "Tools" ('right-hand-side' of page/screen).
Is that what you are asking about?
That tool is used to "Merge"/"Combine" when the "System" DOES NOT recognise them as "Possible" Match.
And, here is a "Knowledge Article" in 'FamilySearch':
How do I merge duplicates in Family Tree by ID?
I hope this helps.
If you know the person is exactly your person but with a different name; and sources show the different names.
You can add AKA (Also Known As) name in the ancestor's person page.
So this is someone's person page. On the right under Details then Life Sketch then Vitals is Add Information. Click on that and add your ancestor's other name/names.
I hope this helps.
as Anitra points out all the variant names for the single person that they used across their life - can all be added as different "alternate names".
you can add as many alternate names as you wish.
merging is only when there are different records that (already exist) in FS FT that you have found that actually are the same person - but duplicated under different names.
but if you are simply asking how to enter the data from scratch - when different names were used over time - thats a single record - with many alternate names that you can add.
I think what you are describing is common for many immigrants - records having different names for the same person. I believe the previous answers are correct. I haven't often created a person from a record - i.e. linked them in the Tree from a Source - I usually like to Add them manually, see if the Tree recognizes them as already existing and of so select them - I think this way prevents some duplication or the need for later merges - however it seems there is always that person created from a record that isn't recognized and needs to be searched for records and merged manually or by FSID.