MariaDB and MySQL versions equivalence
MariaDB-10.2.2 InnoDB merged from 5.7.14
MariaDB-10.2.35 Merge new release of InnoDB 5.7.32
Lots of miscellany features have been merged at random times.
MySQL 8.0 and MariaDB 10.5 are quite different.  Still, you may not notice any differences unless you reach into rarely-used features.
The two play leapfrog.  For example.
MariaDB had CTEs and Windowing functions in 10.2, long before they were added in MySQL 8.0.
On the other hand, MySQL led the way with JSON and some functions (eg ST_distance_sphere).
Better links:
https://mariadb.com/kb/en/function-differences-between-mariadb-and-mysql/
https://mariadb.com/kb/en/mariadb-vs-mysql-features/
And there maybe more.  Note: it is in MariaDB's best interest to list the differences; MySQL prefers to mostly ignore the existence of MariaDB.
Errors, issues, bugs -- They look at each other's bug list, then add fixes as they see fit.
Syntax - Not much difference (except for CTEs, see above)
Features and behavior - MariaDB moved toward standards more aggressively than MySQL.  For example, in a subquery with an ORDER BY, the ordering became ignored in MariaDB long before in MySQL.  ONLY_FULL_GROUP_BY is another point of difference.  In both of these examples, the &quot;new&quot; version is &quot;more correct&quot;, though incompatible.
The Optimizers have diverged.  I deal with performance a lot, but I would say only a few percent of queries show a noticeable difference between the products.  And the difference could be in favor of either product.
If you are just starting, don't worry about which one you pick; you have a long learning curve before you get into the esoteric things I have listed.