Plugins migration

while migrating to latest from 3.8,
if any plugin is not there in latest version can i use the same old plugin in new version?

No you need the Plugin version to match nopCommerce version.
Upgrade the source code
Check on the Marketplace for the latest version
or contact the original plugin developer

ok thank you..
but i think that we have a plugin as an api to our mobile app.
we can use the same by changing the code to .net core
is it ??

Yes if you have the source code.
Use one of the existing plugins as a template and change/add you functionality
There will be quite a few changes including Database and Async processing

theme urban is used in our 3.8 nopcommerce source.

my doubt is I have to download new urban theme (127$) or can i use the same used in 3.8 if i have the full source.

target nop ver:4.3

It will take you many weeks if not months to convert the theme and any plugins and test and debug it
How much is your time worth ?
I would be buying a new version of the Theme and plugins.
Then its done instantly.

Why target v4.3 when v4.5 is so much better ?

thank you brother.

If I am using the new urban theme,
step 1: i will copy the plugins files in plugins folder and theme files in theme folder

i have a backup of my old urban theme cstml files and css with customization, (nop 3.8)

so after step 1 , suppose if have changes in home index file, can i replace the new index file by the old cstml? ??

if no i think i have to do the same changes in new index file instead of replacing it fully.

i need clarification on this also pls

Yes it is best to use the new plugin and theme and incorporated any changes that are still required from your existing .cs and .cshtml files