How to maintain positions in search engines when redesigning a website?

Question

Search Engine Optimization

Hello. We had a site, some pages of which are in the top of the issue. Reworked the structure, i.e. everything has completely changed: CMS, uruls. How now to competently transfer the domain to a new site and not lose positions?
Suppose there is a list of pages of the old site
/page1
/page2
/category/product1
and the corresponding pages of the new site
/p1
/p2
/shop/prod1
it will be enough to register 301 redirects of the type (there will be about 150):
Redirect 301 /page1 /p1
Redirect 301 / category/product1 /shop/prod1
And attach the old domain to the new site? (maybe you need to write it differently, through regular expressions?)
Task 2: there used to be 2 sites, now they have been merged (again, a complete redesign), the links on the new one have changed. Would it be correct to do the
Redirect 301 /page_old_site1 /page_new_site1
Redirect 301 /page_old_site2 /page_new_site2 redirect in the same way
and attach one domain to the new site as the main one, and the second one as an alias?
Do I need to touch robots.txt in these cases?
Thank you.

Answer 1

save relevant content and make redirects

Answer 2

it will be enough to register a 301 redirect

Yes
no, well, or a symbiosis of a redirect and a regular expression. The main thing is that the old url gives a redirect to the new one.

Answer 3

There was a project: The
old site of 5000 products in one structure was transferred to a new site completely with a different structure.
At first they were looking for a way to do something with the structure and regular expressions, then they realized that everything had gone too far.
We made 5,000 separate 301 redirects from the old product to the new one at the site level - the positions remained, the pages quickly moved to the right ones in the search.
ps not an Indian, there really was no other choice.